Category Archives: Electrical Topics

AC Electricity Fundamentals – Part 1

About this article

Initial post: 2/14/2019

This article discusses the concepts and terminology of AC electricity at an introductory level. The scope of the article is limited to the AC power systems found in North and Central America. In Part 1 (this part; already plenty long enough), I will discuss the basics of AC power generation and the delivery of AC power to single-family residential neighborhoods and homes. In Part 2 I will continue with a more focused discussion of the AC power systems on cruising boats.

I chose this approach for two reasons. First, almost all homeowners have some familiarity with household AC electricity. At the very least, most homeowners can find the circuit breaker panel and reset tripped breakers. Second, and more important, boat AC electrical systems are just a subset of what is found in a single-family residential AC installation. Boat AC systems are equivalent to sub-panels in a residence. Sub-panels are subordinate to the main service disconnect panel in a residential building, and in the same way, boats are subordinate to the AC electrical infrastructure of a marina. A basic understanding of household AC electrical systems puts boaters 75% of the way towards understanding boat AC electrical systems. Where boats differ from land-based residential buildings, the reasons are based on specific safety issues that emerge in, and are unique to, the marine environment. Boat AC electrical systems are significantly more complex than single family residences.

This article will assist readers in having confidence to talk about electrical topics with a professional, marine-certified, electrical technician, either designer or tradesman.

Safety

There is one absolute, always rule whenever you must deal with electricity. VIRTUALLY ALL ELECTRICIY CAN BE DANGEROUS TO PROPERTY AND LIFE. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.  The large batteries found on boats can produce explosive gasses and store enough energy to easily start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! If you will be working in noisy environments, with running engines or other loud machinery, WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!

The rule is, “if you aren’t sure what to do and how to do it, stop. Don’t do anything until you’re sure of the “what,” “how” and “why!”

Electrocution

Electrocution is a biological insult that starts with an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death.  Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electrocution, there are several things to do immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 9, below:

  1. STAY CALM!  You can not save someone else if you panic!
  2. Avoid becoming a victim yourself!  DO NOT TOUCH THE VICTIM, METAL MACHINERY OR NEARBY METAL OBJECTS IF POWER IS STILL PRESENT!
  3. SCREAM FOR HELP! ATTRACT ATTENTION!  Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution.”
  4. REMOVE THE POWER SOURCE FROM THE VICTIM BY DISCONNECTING THE ELECTRIC POWER at the pedestal.
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK; after power is removed, raise the face of an unconscious victim out of the water.
  6. DO NOT ENTER THE WATER YOURSELF!  Do not “go” unless and until YOU KNOW FOR CERTAIN that electric power has been turned “off,” AND YOU ARE NOT ALONE ON THE SCENE.
  7. Once power is disconnected and the victim’s airway is  secured above water, if help has not arrived, call 911 again!  Two 911 calls are better than none.
  8. With power disconnected, and with access to the victim, evaluate and initiate CPR as appropriate.  CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.
  9. CONTINUE CPR UNTIL THE VICTIM REVIVES, UNTIL EMS ARRIVES TO RELIEVE YOU, OR UNTIL YOU ARE PHYSICALLY UNABLE TO CONTINUE!

Basic Electrical Working Concepts  – Volts/Amps/Ohms

Like gravity, electricity is invisible. A common analogy used to explain electrical concepts is to liken an electric system to a community water system. Consider the familiar garden hose fit with a nozzle. In the garden hose, when the nozzle is opened, “pressure” in the system makes water flow.

In this analogy, the water in the hose is analogous to electrons in a wire. “Voltage” is the “propulsive energy,” or “pressure,” that makes electrons flow through the wire. The greater the water pressure, the more water flows per unit time. Similarly, the more voltage that is present across a circuit, the more electrons will flow through the circuit per unit time. The amount of water that comes out of the hose is measured in “gallons.” The flow of electrons through wire is measured in “amperes,” or “amps.”

In a water hose, the nozzle restricts the flow of water through the hose. The flow of electrons is restricted in electrical circuits by the electrical property of “resistance.” All materials that conduct electricity have some amount of resistance. Silver and gold have little resistance per unit length. Pure copper has only slightly more, and aluminum has slightly more again. Even the small resistance of a copper wire is extremely important in power distribution applications. Electrical “resistance” is measured in “Ohms.”

Assume we have a 3” diameter water hose and a 1/2” diameter water hose, both attached to the same water source. Only so many molecules of water can fit through the small hose in a minute, but many more molecules of water can fit through the large hose. This concept is called “carrying capacity.” Only so many electrons can “fit” through a wire per unit time.  The larger the wire, the more electrons.  Electrical “carrying capacity” is called “ampacity.” “Ampacity” is a rating assigned to wires.  Wires of the same metal, of different sizes and covered by insulation with different thermal and chemical properties, have different rated “ampacities.” The ampacity rating is the safe maximum current the wire can carry within the temperature rating of the wire’s insulation. Ampacity tables are widely available on the Internet.

Ohm’s Law – Memory Aid

The mathematical relationship between voltage, current, resistance and power is defined by “Ohm’s Law.” Ohm’s Law is probably the most fundamental relationship there is in the entire realm of electricity.  Folks who deal with electricity regularly have this relationship emblazoned in their brains, but for the rest of us, this “memory aid” is  extremely helpful! First, decide what variable you want to calculate. It’s unusual not to know at least two of the necessary variables. For example, today I saw a TV advertisement for a small, portable, plug-in electric space heater. The device plugs into a 120V outlet, so we know E = 120V. I went to the website and found that the unit is rated at 600 Watts, so we know P = 600. For use on the boat, I wondered how much current the device would draw. From the two known variables, we can calculate that the unit will draw about 5 Amps of AC current, which indeed may be OK for some uses on a boat. We also know the unit’s equivalent resistance is 24 Ω (“Ω” is the Greek Letter “Omega,” and is used as shorthand for “Ohms.”)

Ground

The Earth – the crust of our beloved home planet – is electrically conductive. It has many minerals and mineral salts which provide “free electrons.” In the presence of a voltage, electrons flow from point-to-point around and within the earth’s crust. By far the most dramatic example of this is the natural phenomena called “lightening.”

The electrical potential of the earth is defined to be “zero” volts. It is the standard reference point for shock and electrocution safety. In order to connect a residential electrical system to “earth ground,” one or more interconnected rods of copper are driven into the ground. The neutral return point of the residence’s electrical system is physically connected to the network of copper grounding rods.

The concept of “earth ground” is absolutely essential for the safety of people, pets, farm animals and wildlife.  The entire electric distribution grid of the country is connected at innumerable points to rods driven into the earth (the “electric grid” is a “multi-earthed system”).  Every residential property has an “earthing” connection at the service entrance to the home.

The essential point here is that “earth ground” is a universally understood reference point for all power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the total absence of any voltage.  We will return to this concept over and over as we proceed in our discussion.

Circuit Common/“Common”

The concept of “earth ground” is essential for electrical safety, but an earth ground is not necessary for electric circuits to operate. The term “common” is useful in electrical design. It is used among power distribution engineers and craftsmen to reference the conductor that returns current flowing in a circuit from the load to the source.  This is the purpose of the “neutral conductor” in AC electric systems. This conductor does not have to be “0” volts with respect to ground. The “common return” is a “free-floating” conductor. It is extremely important to understand the difference between the concepts of “ground” and “common.”

The term “common” or “circuit common” is not often used in routine conversation.   The common return of a circuit is frequently – colloquially – called its “ground.”  The most appropriate term in household electrical systems is “neutral.”  “Neutral” is a specific term that refers to the current-carrying return conductor of residential AC circuits, but it is not a specific reference to “earth ground.”

Direct contact with energized high voltage is completely safe as long as you are not “across” two or more electrical conductors. For current to flow, there must be a connection between two conductors where there is a voltage difference between them (that is, “across a voltage”). Consider, birds sitting on high tension transmission lines, or squirrels running along neighborhood overhead wires. They are safe because they are on, but not across, a voltage. The animal’s entire little body is raised to the voltage of the wire upon which they sit, yet they are perfectly safe because there is no path for current to flow THROUGH the body. The electrical activity of their brains and hearts is not affected. But, a human being on a wet concrete floor wearing leather shoes best not come into contact with a “hot” wire. That concrete floor is made with salt-containing minerals, and most definitely is electrically conductive, especially when wet. A person standing on that floor and simultaneously touching an energized wire is “across” an electric voltage. That is a shocking experience!  Maybe, a fatal, shocking experience.

“Conventions” vs. Facts

Within the study of electricity as a science, there are hard electrochemical and materials facts, and then there are shorthand ways people talk to each other about complex concepts.  This happens in all professions, of course.  It’s all fine until the terminology confuses an understanding of the true concepts.  Some examples:

  1. It is a fact of physics that electrons carry a negative electrical charge, which means electrons flow from a more negative voltage in a circuit to a more positive voltage.  However, by universal agreement, or “by convention,” the entire practice of electricity and electronics treats current as flowing from positive to negative.  The direction of electron flow has no practical importance, but to properly interpret electrical diagrams, you need to understand the conventional way current flow gets represented by arrow-containing symbols.
  2. The symbols on electrical drawings are all agreed by “convention,” or “working agreement.”  Industry-specific symbols are agreed by international standards organizations.  Where there are symbol differences, their meaning is often obvious.  Some differences occur across international boundaries.  The power industry uses different symbols than are used in the electronics industry.
  3. The “single phase, center tapped, three wire” service is the residential standard in use, by convention, all across North and Central America.  It is institutionalized in the National Electric Code of the US and The Canadian Electric Code in Canada.  Completely different systems are used in other parts of the world, including Europe, Asia, Oceania and southern South America.
  4. The insulation used to coat electrical conductors is colored.  The colors, by convention, identify the use to which wires are put.  Understanding the color schema for wires is essential to electrical safety.  Mistakes here can be fatal.  The meaning of colors vary from country to country.  There are numerous differences between the United States and the nations of the European Economic Community and Oceania.  For those interested, tables are available on the Internet that document color meanings.

Science and Craftsmanship

The laboratory study of “electrical energy” is a theoretical and conceptual science.
Electrical craftsmanship is practical.  I will discuss only a tiny subset of the technical terms and concepts that are necessary to understanding low voltage AC as found in residential and boat applications.  Craftsmanship involves selecting materials, employing fabrication techniques, installing and maintaining electrical equipment, all with the goal of accomplishing some intended design purpose.  Craftsmanship is performed by electricians or electrical technicians and governed by formal regulatory controls called “electrical building codes.”

I view craftsmanship in two stages, which can be sequential or iterative.  If you have ever done an electrical project, you’ve performed both of these functions.

The first stage is the domain of the “circuit designer;”  i.e., the person who designs a branch circuit for installing a ceiling fan with a single switch to turn the fan “on” and “off.”  Or a slightly more complex branch circuit with three switches to turn a light “on” and “off” from different locations.  Or a much more complex array of multiple branch circuits to power a “man cave” or “she shed.”  Or the system for an entire home.  The designer must have solid knowledge of the National Electrical Code (NEC).  Electrical designers for boating applications must be thoroughly familiar with the American Boat and Yacht Council (ABYC) electrical standards.  The NEC and ABYC standards have as their purpose avoiding or minimizing present and future loss of life or damages to property.  The work product of the designer is a system drawing that defines the purpose of a circuit and the manner in which that purpose will be achieved through the use of electrical equipment, components and materials.  The work product includes a the bill-of-materials of the components required to implement the project.  For most projects, a reliable cost estimate can be produced at this stage.

The second craftsmanship stage is the domain of the skilled technician who is charged with the doing of the thing.  This craftsman must know how to use and interpret the designer’s drawings and how to use an enormous array of electrical meters and mechanical tools in the safe fabrication, construction, installation and maintenance of electrical circuits.  This craftsman must understand current assembly techniques, materials and supplies, and must understand and deeply respect industry safety practices.  Safety practice involves knowing when to and when not to work around, and with, energized electrical circuits.  On boats, because of the special safety implications of an electrical system on a floating structure, this craftsman must understand not only what to do and how to do it, but in fact, why things are done as they are, in making an electrical installation safe.

Key Concepts and Terms

  1. Ohm’s law – describes the mathematical relationship between voltage, current, resistance and power.
  2. voltage – (Volt) the quantification of “Electromotive Force” (EMF) (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive Force is measured across two points in a circuit.
  3. current – (ampere; amp) a quantification of the number of electrons flowing through a circuit at any one time.
  4. resistance – (Ohm) a characteristic of an electrically conductive material that tends to retard or impede the flow of electrons through it.
  5. power – (elect: Watt; Joule) (mechanical: inch-pounds, foot-pounds) elect: the amount of “work” that electricity performs in its application.  In purely resistive applications, light or heat.  In turning a motor, torque.
  6. frequency – (Hertz) the number of times a wave goes through a complete cycle in a standard measurement time interval, usually one second.
  7. ampacity – (Amp) a rating of the ability of a conductor of given material, diameter and insulation properties to conduct an electric current within the temperature limit established by the properties of the wire’s insulation characteristics.  (current: amperes; amps) (temperature: degrees Centigrade)
  8. source – the origin from which AC power emerges to energize a circuit.
  9. load – the components of an electric circuit where energy is consumed to do useful work; “useful work” includes production of heat, light, or torque via a motor.
  10. common – a portion of a circuit connection or set of connections that creates a direct return path for electrons flowing in an electric circuit.
  11. neutral – a special case in an AC circuit of a non-ground return path for electrons flowing in a North American standard residential electrical service.
  12. ground – a universal  standard earth reference voltage of “0” volts.
  13. fault current – an abnormal path for current flow, usually to ground.  Fault currents represent potentially dangerous conditions.
  14. short circuit – a specific category of electrical fault resulting from an unintentional direct connection of an energized conductor to either a return circuit or an earth ground.  This low-resistance, unintentional connection results in the flow of extremely large fault currents, and causes overload protection devices (fuses, circuit breakers) to open in order to disconnect the energized power source.
  15. GFCI (Ground Fault Circuit Interrupter) – an anti-shock safety device that senses leakage currents and disconnects the energized power source.
  16. AFCI (Arc Fault Circuit Interrupter) – a fire protection safety device that senses lose connections and disconnects the energized power source.
  17. GFP/EPD/ELCI (Ground Fault Protection/Equipment Protective Device/Equipment Leakage Circuit Interrupter) – similar to GFCI, but higher disconnect specifications.
  18. chase, raceway, conduit, “emt” – enclosed containment spaces in a building or a boat through which wires are run to achieve access to distant locations or to protect wiring from accidental physical damage.
  19. Field Coil – the rotating part of one design of AC generator; this coil can be a DC permanent magnet (typical in small machines), or a DC electromagnet.
  20. Voltage Regulator – the device that determines the strength of the magnetic field in an AC genset by adjusting DC current flowing in the spinning field coil.
  21. Stator – the fixed coils of one design of AC generator, from which sine waves of AC power emerge.
  22. Armature – the power-producing component of a generator; the rotating part of a DC generator; the fixed coils (Stator) in one design of AC generator.
  23. switchgear – a generic term for all disconnecting devices (fuses, circuit breakers, switches, power panels).  This term is used across the electrical power industry, from generating station to transformer yards to residential locations.
  24. Inductance (Ohm)/capacitance (Farad)/Power Factor (unitless) – technical characteristics common to the behavior of AC electricity in circuits that significantly affect large motor driven appliances and all electronic devices.  These become increasingly important as voltages, frequencies and power consumption rise.
  25. Managing the collapse of a magnetic field – a design consideration of any magnetically operated electrical devices (motor, generator, relay, etc), and many solid state devices.  A significant safety consideration for maintenance craftsmen.  When a magnetic field collapses, it creates a very high energy spike, which sometimes includes an electric arc.
  26. ABYC – American Boat and Yacht Council, Annapolis, MD.  This organization produces a very comprehensive set of electrical standards applicable to boat manufacturers, the marine insurance industry and boat owners.
  27. NFPA – National Fire Protection Association; owner/creator of the NEC.
  28. NEC/CEC – National Electric Code (USA), Canada Electric Code (Canada).  electrical design standards for political subdivisions and the construction industry.  Ranges from codes for residential housing, light commercial and industrial buildings, elevators, hospitals, airports, and heavy industry.

Generation (Source) and Consumption (Load)

There are three primary divisions of all electrical power distribution systems, including the global system we call the “nationwide electrical power grid.”  They are 1) the source of the electrical power, 2) the transmission system, or interconnecting wires and switches that carry power from the source to the point where it is consumed, and 3) the load, or the part of the system where the electrical energy is transformed into useful work.

At the level of the US national electric power grid, the source of AC electric power is one or more generating machines located in one or more generating stations.  Often, the term “alternator” is used interchangeably with the term “generator.”  These generating stations range in size from enormous, industrial-sized installations to small rural hydroelectric dams to units suitable for individual residential applications.

The substations, switchgear and wiring that connects sources of power to load centers are extremely complex, involving may hundreds of miles of high tension power lines, enormous transformers and highly complex switches.  Transmission equipment  can also be as simple as an extension cord run from the garage to the hedge clipper.

Electrical loads fall into the entire range of electrical equipment, from the largest commercial synchronous motors to the smallest and most humble LED electric clock.

About AC Generators

Typical AC electric generators have a rotating magnet (imagine the big bar magnet you played with in grade school science) which has a north pole and a south pole.  That magnet may be driven by a belt, wind or water turbine or direct drive, but ultimately, it’s a spinning magnet mounted on a shaft.  The north and south poles of the spinning magnet travel in a circular path.  A pick-up coil is positioned just outside the edge of the circle.   As the magnet spins on it’s shaft, the poles of the magnet approach the fixed pick-up coil, producing an electric voltage at the pick-up coil.

As the spinning magnetic pole gets physically nearer to the pick-up coil, the voltage at the pick-up coil gets progressively larger.  Once the magnetic pole moves past and away from the pick-up coil, the voltage at the pick-up coil gets progressively smaller again. When the north and south poles of the magnet are both equally distant from the conductors of the pick-up coil, no voltage is produced at the pick-up coil.

The voltage induced in the pick-up coil by the passage of the north magnetic pole is equal in magnitude and opposite in polarity from the voltage induced by the passage of the south magnetic pole.  One pair of north and south magnetic poles that sequentially rotate past the pick-up coil produce one cycle of AC voltage at the pick-up coil on each revolution. The speed, in revolutions per minute (RPM), of the spinning magnet determines the frequency (Hz) of the generated voltage.  The resulting AC wave form is called a “sine wave,” which is centered around “0” volts.  Sine waves rise and fall in smooth, graceful fashion with no sharp transitions in the shape of the wave.

In the preceding diagram, there is a geometrically balanced arrangement of a spinning magnet and a geometrically balanced arrangement of pick-up coils.  The output power of the generator is directly proportional to the strength of the magnetic field, up to the limits of its materials and mechanical design.  The output consists of two wires, and is referred to as “Single Phase” AC.

Many physical arrangements of the magnet poles and pick-up coils are possible, but the basic principle is the same for all AC generators.  To produce 60Hz AC, a single phase, two-pole, gasoline-driven, big box store genset (2500W to 6kW) typically spins at 3600 rpm; a single phase four-pole Marine genset (7.5kW to 25kW) typically spins at 1800 rpm. Because of the enormous weight and mechanical forces involved, multi-megawatt commercial generators may have 24 poles and spin at 200 rpm.

The rotating magnet in an AC generator is called the “field coil.”  The field coil is just a spinning DC electromagnet.  DC is fed to the field coil via slip rings and brushes on the spinning shaft.  The fixed pick-up coil in an AC generator is called the “stator coil.”  It is wrapped around iron support columns that are fixed in position around the perimeter of the frame of the machine.

Occasionally, the term “armature” may be heard; an “armature” is defined as the power-producing component of a generator.  In a DC machine, it is the armature that spins, with field coils stationary in the frame of the machine.  Fixed field coils with a spinning armature is a construction alternative for small frame AC alternators (<25kW).  This is both more costly to build and much more complex mechanically, so not common in the generator sizes found in the consumer retail market.

The amount of power that a generator can produce depends on many aspects of the physical construction of the machine, the amount of energy available from the driving motive source, the size of the internal conductors and underlying metal components, the strength of the internal magnetic field, and many other factors.

“Single-Phase” and “Three-Phase Power”

In reading through questions and discussions on various Internet boating bulletin boards , the differences between “single phase” AC and “three-phase” AC is often a point of confusion.  Three-phase power is extremely rare in residential settings, and few people have any life experience with it.

Consider the above preceding description of generator concepts.  Commercial power plants are fit with enormously large and heavy generators.  For several reasons, it is advantageous for these very large machines to spin slowly.  These generators are built with a large number of physical pick-up coils.  These pick-up coils are arranged as pairs in sets of three.  Logically – not physically – the machine appears as shown in this diagram.  These sets of pick-up coils are placed around the perimeter of the circle of the rotating magnet, at geometric intervals of 120° around the 360° circle.

As the bar magnet spins, the voltage in each of the pick-up coils rises to its positive maximum, falls back to zero, then rises to its negative maximum, and falls back to zero. This happens in each set of coils, in turn.  The result is three sinusoidal waveforms being produced by the same rotating magnet (“field”).  The three wave forms are displaced in time by 1/3 of a cycle (120°) of rotation of the rotor.  Enter here, the short-cut language the electrical industry has for this: “three-phase AC,” often shown on electrical diagrams written as “3-ϕ” or “3-phase.”

For commercial power plant operators and distributors, three-phase power is far more economical to generate than single-phase power.  Worldwide, all commercial electric power is created in generators configured as 3-ϕ machines.  The phases are designated as “Phase-1,” “Phase-2,” and “Phase 3;” this terminology can also be “Phase-A,” “Phase-B,” and “Phase C.”  Three-phase derived power is of special interest for boaters with 120V/240V, 50A shore power connections since it results in 120V/208V voltages.  More details in the section on “Special Situations.”

Single-phase AC is the type of electric service found in virtually all single family residential applications because it is easily derived from three-phase distribution systems, in two ways.  The first is to connect a load between any one of the phases of a three-phase service and a suitable electrical return point, usually the common of a 3-phase wye configuration.  This is how residential neighborhoods are serviced.  The second way to obtain single phase AC is to connect the load between any two phases of the 3-phase distribution system.  This is common in commercial applications and in apartment and condo buildings, but not in single family residential services.

Residential Neighborhood

For simplicity, I started with power as delivered to single family suburban homes excluding light commercial buildings.  Light commercial buildings (condos, townhouses, apartments, offices, stores, and marinas) can all be served with single phase AC electric service, just as single family residences are, but more commonly, they are served by 3-phase utility service.  I will talk about these buildings later in the section on “Special Situations.”

Utility company power transformers have input sides, called the “primary,” and output sides, called the “secondary.”  Physically, both the primary and the secondary coils of the transformer are independent windings of wire wound around an internal metal core.  The windings are electrically isolated from each other; i.e., “insulated” from each other, but are “coupled” to each other by a shared magnetic field.  As the incoming primary voltage rises and falls, the magnetic field in the metal core strengthens and weakens.  As that magnetic field strengthens and weakens, voltage appears at the secondary.

Residential Single Phase :Street” Transformer (Typical)

The “load” for the transformer outside your house usually consists of four or so residential homes.  Throughout North and Central America the transformer is matched to the primary voltage to produce 120V/240V at the secondary.  The utility company transformer reduces the primary voltage to the residential requirement.  The range of transmission system primary voltages in a three-phase grounded wye configuration include; 34,500/19,900 volts; 22,900/13,200 volts; 13,200/7,620 volts; 12,470/7,200 volts; and, 4,160/2,400.  The first number in these number pairs represents the phase-to-phase voltage; the second number represents the phase to neutral voltage.  A single phase primary in a residential neighborhood is most commonly 7,200 volts, measured phase-to-neutral.  In rural residential primaries, 13,200 volts is common.

Transformer coils can be built with one or more “taps” on both the primary and secondary windings (coils).   The secondary winding of a residential power transformer is built with a single tap at the electrical midpoint of the coil.  This configuration is called a “center-tap.”  The three wires that come to a single-family residential  home from the utility pole are the two end-points of the secondary coil and the center-tap.  That center-tap conductor becomes the “neutral” within the building’s distribution wiring.

In the world of the electrical craftsman (electrician), it is desirable practice in a residential building or boat to have about ½ of the total household load attached to each side of the service transformer.  This practice balances the load on the secondary windings of the transformer on the street, and balances the concentration of heat that builds up within the windings and metal core of the transformer.  Transformers are oil cooled, and under heavy loads, they can get very hot.  Thus, balancing heat dissipation is crucially important in periods of very high electrical demand.  Days that are 104°F on the Chesapeake Bay or -30°F at International Falls are not times you’d want the transformer that serves your home to fail!

The definition of a North American residential standard power distribution system is a “single-phase, center-tapped, three-wire” service (alternatively, “single-phase, center-tapped, three-pole” service; in this case, the term “pole” represents a current carrying conductor.  Other common terms for this systems include “grounded neutral” and “split phase.”  The three parts of this definition are:

  1. single phase
  2. center-tap (gives rise to the system “neutral;” “grounded neutral”)
  3. three wires (two “hot” and one “neutral”)

From time-to-time, professional electricians and DIY lay technicians incorrectly refer to the residential “single phase, center tapped, three-wire” configuration as consisting of two phases.   The “evidence” is that one leg, “L1,” is 180° out-of-phase with the other leg, “L2.”  While “true,” this misleading factoid is a measurement curiosity caused by performing the electrical measurement from an inappropriate reference point.  Voltages from the two halves of our residential service will appear to be out-of-phase if measured with an oscilloscope from Neutral to “L1” and then from Neutral to “L2.”  The false appearance is the result of looking at the secondary of the transformer with reference to its center tap rather than across the entire winding.  This measurement curiosity is not present if the secondary is measured from “L1” to “L2” (or vice versa).  Think of it this way.  There is only one magnetic field alternately rising and falling in the transformer, driven by the rise and fall of the single-phase input at the primary.  That is the defining characteristic of “single-phase” equipment.  In a three phase device, there are three independent magnetic fields rising and falling within the equipment.  That is the defining characteristic of 3-phase equipment.  This distinction becomes extremely important when describing rotational torque in a 3-phase motor.

The above discussion is somewhat of a “technicality” issue, which has no practical importance in real life, and can safely be ignored!  When I was a pup, and first worked for an electrician in the early 1960s, I learned to refer to the two residential “hot” lines as “legs” instead of “phases.”  Doing so distinguishes the in-residence wiring from the conductors of the utility distribution system.  Frankly, except for the concepts involved, it’s really not important how you refer to this as long as you don’t let it confuse you!

Service Entrance – Single Family Residence

So now we understand that the electrical service entering a single family residence is a “single-phase, center tap, three-wire” service.  In our single family residence, if there are overhead wires and utility poles in the street, the three wires coming from the transformer are routed to a weather head or anchorage on the home, where they are spliced to wires leading to the electric meter.  In most jurisdictions in the US, the wires coming from the street are owned by the utility company.  The weather head, meter box and the wires from the “street splice” to the meter box are customer-owned.  The meter itself is owned by the utility company.

The customer-owned wire to the electric meter and from the meter to the main disconnect panel inside the building is comprised of two insulated wires (usually black) surrounded by a wrapping of bare wire strands.  This entire cable assembly is insulated as a single triplex unit.  This cable has a flat rectangular cross-section and is known as “Type SE,” or “Service Entrance” cable.  The two hot lines are routed to the input side of the “main” circuit breaker in the main disconnect panel.  The uninsulated neutral wire of the Service Entrance cable is routed to the neutral buss bar in the main panel.  The neutral buss bar is insulated from everything else in the service disconnect box, including the metal of the box enclosure, itself.  If the residence has an underground service, wires from a transformer located on a ground-level concrete pad will all be individually insulated wires rather than a triplex assembly.  They will be routed through underground conduit into the electric meter and then to the service disconnect panel.

The output side of the main circuit breaker in the disconnect panel is attached directly to metal “buss bars,” to which individual branch circuit breakers are fitted.  These buss bars are referred to as “L1” and “L2,” because they are on the overload-protected load side of the panel’s main circuit breaker.  The input side of the main disconnect breaker is referred to as the “Line” side and the output side is referred to as the “Load” side.

What we have not yet discussed is the “safety ground” that is required throughout the residence by the National Electric Code.  This safety ground attaches to every outlet, switch plate, ceiling fan, luminary fixture and appliance in the residence.  In a residential application, there are one or more copper rods driven into the ground outside the building.  The grounding wire is usually of bare #6 or #4 AWG stranded copper wire, and is routed from the buried ground rod(s) to a buss bar located in the service disconnect panel.  That buss bar is physically mounted on, and electrically connected to, the service disconnect panel’s metal box enclosure.  All of the ground wires that come from outlets and appliances everywhere in the building will be routed to this buss bar.

Main Service Disconnect Panel

We know from earlier discussion that the “neutral” in the building is a free-floating return line for power that arrives from the transformer hot lines.  But a free-floating return point is unlikely to be at “zero” volts, which is required to avoid electric shock in the home.  The NEC requires that the neutral line in a residence be bonded to earth ground “at the derived source of the electricity.”  For a home, the “derived source” is defined to be the main service disconnect panel.

In one design of main service disconnect panel, there is a buss bar dedicated to collecting branch circuit neutral conductors and a physically separate buss bar dedicated to collecting safety ground conductors.  In this style panel, there is a screw – usually dyed green in color – in the neutral buss bar. That screw is the “system bonding jumper,” or “bonding screw.”  This design allows the panel to be used either as a main disconnect panel or as a sub-panel.

If the disconnect panel is to be used as the Main Disconnect Panel, the bonding screw must be seated into the panel’s metal enclosure housing to electrically “bond” the “neutral” buss bar to the “safety ground” buss bar.  That screw is not for any mechanical purpose; it is the electrical bridge that make the “neutral” to “earth ground” connection.  THIS IS A CRITICALLY IMPORTANT SAFETY FEATURE.  NEVER OMIT OR REMOVE THE BONDING SCREW!

Sub-panel(s)

Sub-panels, a special case of residential switchgear, are used for several reasons:

  1. reduce the number of wire runs from the main service disconnect panel,
  2. manage the round trip length for long branch circuit wiring runs,
  3. manage the number of wires run in hidden chases/raceways/conduits, and
  4. reduce the cost of the installation.

The NEC does not limit the number of sub-panels that may be installed in a residential electrical system. Larger residential systems may have sub-panels located in several places around the home; ex: attached or detached garage, detached “guest quarters,” workshop, greenhouse or yard shed, pool house, Man Cave, She Shed, attic-space mechanical service (air conditioning compressor or attic vent fans), etc.  To install a sub-panel in residential applications, a single, appropriately sized 4-conductor cable, “Type SER,” is run from the main service entrance panel to the sub-panel (red arrow, below).  This 4-wire configuration carries “L1,” “L2,” “N” and “G” to the sub-panel switch box.  Because the sub-panel is subordinate to the main disconnect panel, the neutral-to-ground bonding screw is NEVER used in any sub-panel switch box.  By definition, the sub-panel is not the “source” for these branch circuits.  The main disconnect panel remains the “defined source” of the circuit.

The configuration of sub-panels in a residence is exactly analogous to the configuration of a boat attached to a marina shore power pedestal.  Notice the 240V, 3-pole, 4-wire feeder (red arrow) that connects the residential Main Disconnect Panel to the remote sub-panel.  This feeder is exactly analogous to the 240V/50A shore power cord of a boat.  The sub-panel “feeder cable” is “Type SER.”  It contains three current-carrying conductors and a safety ground.  Rather than the flat, rectangular cross-section of “Type SE,” “Type SER” cable features a round cross-section.  A boat’s “feeder cable” (shore power cord) is “Type SO” or “Type SOW,” which are very flexible cords.  Net: a boat looks like a sub-panel to the marina’s shore power system, and that is why the ABYC electrical standard seems so closely aligned with the requirements of the NEC.  Notice also in the drawing that the sub-panel safety ground leads back to the neutral buss in the main panel.  The neutral-to-ground bond is made only at the “derived source,” which is the Main Disconnect Panel.   Likewise, on a boat connected to shore power, there should never be a neutral-to-ground connection anywhere on the boat.  In both cases, the neutral-to-ground connection is made at the “derived source,” which is the main distribution panel in a residence, analogous to the marina shore power system for a boat.

Note: This main disconnect panel drawing shows a single buss bar which is shared by the neutrals and the grounds of branch circuits.  This arrangement is an NEC-compliant variation in a main disconnect panel.  Many main disconnect panels and all sub-panels will have physically separate busses for the neutrals and the grounds.

Branch Circuits

Branch circuits are where useful work gets done in the home.  There are three use cases:

  1. Between legs “L1” and “L2” alone, without “N,” we can power 240VAC, two-wire (two-pole) appliances; for example, the 240V motor of a deep-well pump, 240V baseboard electric heat radiator(s), or a 240V hot water heater.
  2. With “L1,” “L2” and “N,” we can power 240V, three-pole appliances; these appliances require 240V for some internal functions and 120V for other internal functions; for example, an electric dryer, range cooktop or oven; all of these appliances require 240V for the heating elements, but 120V for the motor and control circuits.  Or, central air conditioning system, which requires 240V for the compressor, but only 120V for the control circuits.
  3. Finally, with either “L1” or “L2” alone, and “N,” we can power the entire panoply of 120V, two-pole household loads; oil or gas furnace, dishwasher, incandescent and florescent lighting, computers, printers, routers, wireless telephones, TVs, VCRs, stereo, refrigerator, freezer, microwave oven, coffee maker, toaster, crock pot, waffle iron, blender, mixer, hair dryer, steam iron, battery chargers, shop tools, CPAP, oxygen concentrator, etc; you get the idea!

Branch circuits originate at a circuit breaker located in either the main service panel or a subordinate sub-panel.  Branch circuits feed either convenience outlets or feed into the attachment enclosure of a permanently installed appliance.  For convenience of installation and maintenance, the individual black, red, white and bare wires of a branch circuit are packaged together within a sheath of plastic outer insulation.  Most residential wire sold in big box, hardware stores and home centers is “Type NM,” meaning “non-metallic.” This is often called “Romex.”  “Type NM” intended to power 120V circuits is called “two-wire with ground,” or “two-pole, three-wire.”  “Type NM” intended to power 240V circuits is called “three-wire with ground,” or “three-pole, four wire.”    Another common residential wire is “Type AC.”  “Type AC” has an armored metallic sheath around the individual colored conductors instead of a plastic outer sheath.  “Type AC” is used for furnace controls for LPG and oil burners, hot water heaters and other appliance in an equipment room or basement, as well as when installed in areas exposed to being physically disturbed or damaged, such as workshops or garages.  Carefully match the wire you buy to the application you have, based on NEC and local electrical codes.

In the U. S., the color of the insulation on individual wires is important; “L1” is black, “L2” is red, “N” is white and “G” is uninsulated copper in convenience and appliance circuits, but can be green or green with a yellow tracer when insulated.

Occasionally, you may encounter a wire in a service disconnect panel or a junction box that has a piece of electrical tape of another color  conspicuously wound around it near its connecting end.  In a residential building, you may see red or black electrical tape wound on a white insulated wire, or you may see a piece or white electrical tape wound on red or black insulated wires.

Do not remove these pieces of tape; they are not an accidental left-over!  It means the installing electrician has “changed” the meaning of the base color of the insulation of the wire.  In residences, the most common place to find it is in wall boxes containing switches that control lighting or fans from multiple doorway locations, or wall boxes at the top and bottom of staircases.   If you ever see this, always triple-verify how the wire is actually being used before proceeding or disturbing the connection.

I have spent a lot of time talking about the current that arrives at the load in one of the energized conductors, “L1” and/or “L2,” and returns to the source in the neutral, “N.”  I have not discussed the use of the green ground wire, “G.”  In a correctly wired, normally operating home or boat AC electrical system, the ground wire should never have any current flowing in it.  The purpose of the safety ground wire is to provide an emergency path for current in order to trip the supplying circuit breaker to remove power from a faulting circuit.  By definition, current flowing in a safety ground is symptomatic of an electrical fault condition.  Fault currents originate from the hot line(s), but return to the source in the safety ground instead of the neutral.  This condition is also known as a “ground fault.”  Never use wire covered with green insulation as a current-carrying conductor.

Circuit Breakers

Contrary to popular belief, circuit breakers/fuses do not protect attached loads!  Circuit breakers do not protect TVs, entertainment systems, computers, microwaves, coffee pots, pumps or compressors.  CIRCUIT BREAKERS/FUSES PROTECT THE POWER-CARRYING WIRING THAT IS HIDDEN IN WALLS AND/OR ENCLOSED IN CHASES, RACEWAYS AND CONDUIT THROUGHOUT YOUR HOME OR BOAT!  They protect the WIRING of your home/boat.  This is a critically key concept.

When wires overheat, their colored insulation can melt, exposing the live conductor.  At that point, energized conductors can touch other now uninsulated conductors, and sparks can fly.  Wires in closed spaces, unusually warm spaces, or chases/raceways/conduits warm up more than wires in un-congested, cool, spaces where there is plenty of air circulation. Overheating softens the insulation.  Wires can get so hot that they will literally melt and can weld themselves together.  This process can cause adjacent nearby wood and composite building materials to burst into flame.  So, circuit breakers protect wires from overload, and therefore, protect the insulation from overheating, melting, failing and causing fires.

There are several common types of circuit breakers, and several manufacturers of circuit breakers and compatible service disconnect panels.  Circuit Breakers for 120V circuits are singe-wide; for 240VAC, they are “stacked” or “doublewide.”  Doublewide breakers have mechanically linked operating levers, and must be doublewide so that they can be physically installed in a service panel in a way that allows them to mate to both the “L1” and the “L2” buss bars at the same time.   If one leg of a 240V circuit – say, “L1” – develops a fault that causes the circuit breaker to trip, the mechanical link causes the other leg – in this example, “L2” – to also be disconnected from it’s source.  Never remove the mechanical linkage between doublewide breaker operating levers.

Switchgear on Boats – Residential vs. Marine-certified

Circuit Breakers should be selected based on the size of the wire they protect.  A 15A circuit breaker protects #14 AWG, Type NM cable; a 20A breaker protects #12 AWG Type NM, and a 30A breaker protects#10 AWG Type NM.  These numbers are based on the 60℃ temperature rating of “Type NM” wire.  Wire ampacities are higher with the 105℃ temperature rating of “Type BC5W2” boat cable.

Circuit breakers used for “over-current protection” (OCP) have rating of 15A, 20A, 30A or 50A.  That said, modern, sophisticated circuit breakers actually carry several ratings.  In a true short circuit, an over-current fault can instantaneously be as high as several hundreds of amps.  By arcing, that extreme amount of current can weld the contacts closed and permanently damage the circuit breaker’s contact points, rendering the breaker inoperable.  Circuit breakers and all switching devices carry an “Ampere Interrupt Capacity” (AIC) rating.  AIC is the amount of current the device can interrupt without being damaged by arcing.

Modern circuit breakers can also have multiple purposes.  Besides OCP, one added purpose is “Ground Fault Protection” (GFP) and another purpose is “Arc Fault Protection” (AFP).  GFP breakers contain a circuit that compares the amount of current being delivered in the hot wire(s) to the amount of current returning in the neutral.  Any difference in outgoing and returning current is a “ground fault.”  Household “Ground Fault Circuit Interrupter” (GFCI) breakers are designed to trip “off” if the difference between supplied and returned current is as little as 4mA – 6mA.  “Equipment Leakage Circuit Interrupters” (ELCI) onboard boats – and Equipment Protective Devices (EPD) on dockside pedestals – protect the whole boat, as a sub-panel.  ELCI/EPD are designed to trip “off” in less than 100 mS if the difference between supplied and returning current exceeds 30mA.

Finally, for use on gasoline powered boats and environments of potentially explosive gas, circuit breakers (and other electrical switching devices) must be rated as “ignition protected.”  This means that any internal arcing (sparking) caused by the contacts opening under load must not be able to come into contact with any airspace outside the breaker’s enclosure.  If explosive gasses were able to infiltrate the breaker’s enclosure, the vapors would be able to cause an explosion.  Of course, common residential circuit breakers are not made to the standard of “ignition protected” devices.

In general, in my opinion, it is bad practice to use “big box” and hardware store electrical switchgear equipment, circuit breakers or wire made for residential applications on a boat.  Residential switchgear is not made to withstand humid, salt-containing air, is not suited to the materials properties required by ABYC, and is not equivalent in temperature ratings for the ampacities of given conductor sizes.  NEVER, NEVER use solid core household wire on boats.

Aggregate Electrical Load – Residential Building

“How  much electrical “stuff” can we run “all at once” in our single family residential home?”  This is a key question for both residential applications and boats.  For boaters, it relates directly to discussions about 30A and 50A shore power cords and inlet wiring sizes.

Today, if you have a home of 2000 ft2 or more with an oil or gas-fired furnace, you’ll have a service entrance with at least a 200 amp service capacity.  If your home has electric baseboard heating and/or central air conditioning, it’ll probably have a 400 amp capacity. In the 1960s, we simply didn’t have as much “electrical stuff.”

What does it mean to “have a 400 amp capacity electrical service?”  In a moderate-sized residential building, if the individual capacities of all of the branch circuit breakers in your residential service disconnect panel were added up, there would probably be between 500 and 800 amps of distribution capacity.  For example:

8 – 30 amp double pole breakers for baseboard heating
1 – 50 amp double pole breaker for the range/oven
3 – 20 amp single pole breakers for the dishwasher, washer, and microwave
1 – 30 amp double pole breaker for the clothes dryer
1 – 40 amp double pole breaker for the hot water heater
1 – 40 amp single pole breaker for that great air compressor in the garage
20 or more – 15 or 20 amp single pole breakers for convenience outlets
1 – 50 amp double pole breaker for the air conditioning compressor

Hmmm…   Adds up to 810 amps (+/-) of branch circuit distribution capacity.  Take out the baseboard heating and you still have 570 amps.  However, that service panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at maximum breaker capacities.  Remember, breakers protect wires, so the individual breaker capacity is to protect the wire, not the attachment.  What happens if you exceed the capacity of the 200A/400A main breaker?  Well, in that case you’d blow the main breaker, but without blowing any of the individual branch circuit breakers.  Hmmm…

So to the question, “what does it mean to have a 200A or 400A electrical service?”  A “200 amp service” means that the installed utility-owned drop from the street, the conductors of the ”Type SE,” 3-wire service entrance cable to the electric meter housing, the conductors from the electric meter to the service disconnect panel, the service disconnect panel itself, and the earth ground connection are all sized and designed to operate in a safe manner when handling up to 200 amps for a 200A service, or up to 400A for a 400A service. If you exceed that capacity, that set of essentially unprotected electrical components may fail.  In effect, 200A/400A is the “ampacity” of the unfused and unprotected service entrance feed components.  So even though you have 500 to 800 amps of branch circuit load attachments, if you never exceed a combined aggregate load of 200 total amps, the distribution box will serve you just fine.  If every you do blow the main 200A/400A breaker in the home, have the cause determined by a qualified electrical professional!

Aggregate Electrical Load – Boat

The previous analysis of loading a residence main disconnect panel applies in exactly the same way to boats.  Most cruising-sized boats with 30A shore power will have well in excess of 30A of branch circuit capacity; likewise, boats with 50A shore power will have proportionally more branch circuit capacity.  That power is delivered onto the boat through a (30A)(50A) onboard main disconnect breaker, or compatible ELCI.  As with the residence case, the service distribution panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at their maximum breaker capacities.  If you exceed the maximum main breaker capacity, you blow either the main disconnect breaker, or the Shore Power pedestal breaker, generally without blowing any of the individual branch circuit breakers.

The ABYC requires an AC Main Disconnect Circuit Breaker within 72 inches of the shore power inlet.  Nothing is allowed to be connected ahead of that main disconnect breaker except the actual shore power inlet connector.  Recall, the purpose of circuit breakers is to protect wiring, and in particular, wiring hidden from view, and away from reasonably easy access, and running through spaces containing combustable materials.  The AC Main Disconnect Breaker protects the boat’s main inlet wiring (the boat’s “service entrance cable,” if you will) up to the main distribution panel that serves the boat’s individual branch circuits. Remember, the ampere rating of the disconnect breaker must be matched to the ampacity of the wiring between the power inlet plug and the main disconnect panel on the boat.  In the case of boats, the wiring installed by the boat manufacturer should reflect what the naval architect sped’ed for the boat.  Remember, the wires we’re talking about provide power to the AC circuit breaker panel of the boat, and carry the total aggregate current load for the whole boat.  Sizing shore power cords smaller than necessary could be dangerous.

AFCI and GFCI-protected Protection

Since 2008, the NEC has constantly extended the AFCI requirement to now include all habitable areas of a home, including kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar places.  Some states have modified these requirements when adopting the NEC as statewide regulatory code (building codes of all kinds are done on a County-by-County basis in Maryland).  Check local building codes before proceeding.

Since 1971, the NEC has continually expanded the coverage requirements for GFCI protection. Today, GFCI protection is required in all “wet” locations in residential buildings, which includes bathrooms, outdoors locations, rooftops, crawl spaces, unfinished basements, kitchen countertop areas, sinks, laundry areas, bathtub/shower stall areas, boathouses, locker rooms, pool areas: you get the idea.  On boats, the ABYC requires GFCI-protected outlets in heads, galley, machinery spaces and everywhere on the weatherdeck.

Should you wish to retrofit AFCI and GFCI-compliance into an older home (a good idea), a reasonable approach is to replace the conventional circuit breakers in the main  disconnect panel or sub-panel that serves affected branch circuits with combination AFCI/GFCI-protective circuit breakers.  That way, all outlets served by that circuit breaker are AFCI-protected and GFCI-protected.  Combination breakers are available from many manufacturers for about $35 – $45 apiece (as of January, 2018).  Discounts are available for volume purchases.   On the boat, physically compatible GFCI-breakers are not generally available, so GFCI-protected outlets are recommended.

GFCI-protected devices do present some unintended consequences.  A common scenario is for boaters to use adapters to enable a 30A or 50A shore power cord to use a standard 15A or 20A, 120V GFCI-protected utility outlet on a dock.  This provides power for a fridge, a battery charger, and maybe a reading lamp, for a night or two.

In the case of deteriorated, cracked insulation on a shore power cord lying in the water, a ground fault current could easily be large enough to trip a GFCI breaker, and that fault would not go away over time.  That condition is a true ground fault.  Not all trips are caused by true faults.  Sometimes, electronic components (capacitors and inductors) within the familiar portable computer “power bricks” can cause “momentary” surge currents that can trip sensitive GFCI protection devices.  Insulation breakdown on blower motors, pumps  and air conditioning compressors, as well as aging hot water heater elements, can cause transient power leaks.  Power spikes on power lines can trip GFCI devices.  All GFCI implementations are exposed to false faults resulting in “nuisance” trips.  When attaching to GFCI-protected outlets, it’s a good idea to set all AC breakers “off” first, then plug in, then turn branch circuits “on” one at a time.

For marinas and boatyards, starting in 2011, the NEC has required ground fault protection on new construction docks (except residential, single family docks until 2017).  These devices are called Equipment Protective Devices (EPD), and are also subject to “nuisance trips.”  To reduce the incidence of nuisance trips, the NEC has adopted two accommodations to lessen the occurrence of false trips on docks.  First, the size of the leakage current – 30mA – that would cause a marine pedestal EPD to trip “off” is greater than (less sensitive than) a 15A/20A GFCI convenience outlet.   Second, the length of time (duration of) the leakage current needs to be present – up to 100mS – has been made longer.  Since 2011, the rollout of these EPD sensors at marinas has been slow, but they are beginning to appear in greater numbers, and all boaters should expect to see EPD protection of marine outlets on docks with increasing frequency over the next few years.  The electrical knowledge and skills found among dock staff are unlikely to resolve problems for those who do experience nuisance trips at a marina.  Particularly on holidays, weekends and off-hours, high-school and college summer help are not likely to be able to assist transient boaters.

“Nuisance trips” may or may not mean you have wiring errors or equipment faults on your boat, but the fact is, many boats do have wiring errors and equipment faults that until recently have been silent and non-symptomatic.  Obviously, “troubleshooting” this scenario could be very complicated and time consuming.  If you have the skills to do it yourself, it’ll cost lots of time.  If you hire a marine electrician to do it for you, it’ll cost lots of money.  Either way, it won’t be easy or inexpensive.  It may well be that you just have older switchgear equipment, like a reverse polarity light with a filament that provides a “leakage path” from “neutral” to “safety ground.”  This is not an unsafe condition, but it will trip some EPD devices.  What is nasty about this is that “your boat is at fault,” and that’s precisely what you’ll get from the marina operator.

Special Situations – Life’s Little Complications

There are two types of three-phase wiring configurations: “wye” (or “star”) and “delta.” Three-phase distribution systems are used in commercial facilities and larger industrial facilities.  Within this category, I include condos, townhouses, strip mall offices, shopping centers, marinas and boatyards.  So, consider for example the case of three-phase distribution systems feeding end-user attachments in a condo or apartment.

In our “single family suburban residence” model, we learned the US standard voltages of a “single phase, center tapped, three wire” service entrance would be 240VAC/120VAC.  For many technical and economic reasons, light commercial and multi-family residential buildings are supplied from a three-phase, wye-connected service.  In a wye configuration, a 4-pole, 4-wire distribution system comprised of  “ϕ-1,” “ϕ-2,” “ϕ-3” and “N” is delivered into the building.  What is finally delivered, in turn, to the individual occupancy units is a 3-pole, 3-wire feeder analogous to the single phase street feed.  It is not, however, derived from the secondary of a single phase transformer.  Rather, it consists of any two of the three phases that came into the building, together with wye’s “N.”  As an example, suite 100 may receive “ϕ-1,” “ϕ-3” and “N,” and suite 102 may receive “ϕ-2,” “ϕ-3” and “N,” and so forth.

In the wye configuration, the voltages delivered to individual occupancy suites are not the standard 240VAC/120VAC.  Between “N” and any of the phases, the suite would see 120VAC. But between the two phases, the suite would see only 208VAC.  This service is written on paper as “208V/120V Y,” to indicate the phase-to-phase voltage, “208V,” the phase-to-neutral voltage, “120V,” and the fact that the configuration is a wye connection, “Y.”  This practice is common enough in the US that household appliances built for 208VAC/120VAC are commonly available in retail outlets for condo and townhouse dwellers.

Fortunately, 240VAC/120VAC appliances connected to 208V/120V Y services will usually work. Many are made to tolerate the lower line-to-line voltage.  The downside is, appliance efficiency may be reduced.  The power available across the “L1” and “L2” lines of phase-to-phase connection service will electrically be only 85% of the power available from the full design voltage.  As boaters, we need to be aware that many marinas are configured in this way.  If a boat has 240VAC appliances aboard – air conditioning, hot water heater, range/oven, washer/dryer, etc. – those appliances will receive “low voltage” if the marina is configured to provide “208V/120V Y.”

The most significant impact might be to 240VAC pump and compressor motors.  With a low voltage on the appliance, efficiency will be compromised, and motor overheating might occur.  Three phase “Y” distribution configurations are common in marina’s.  Boats with one or two, 2-pole, 30A shore power connections would not be affected.  Those connections are 120VAC.  Those with two 30A shore power cords connected to a “Y” adapter into a 50A outlet on a pedestal are also unaffected.  That’s because even though you are bringing the two different phase lines aboard, your boat does not have any 208V/240V appliances, so nothing aboard is affected.  Boats that connect to shore power with 3-pole, 50A shore power cord are potentially affected, as that 3-pole, 4-wire connector provides 240VAC with the expectation that it will be used on the boat.  Without 240V appliances, there is no affect.

The only thing you, as a boat owner/operator, can do to protect your appliances is to measure, with your onboard volt meter(s), the line voltages (2xxVAC/120VAC) provided by the shore power pedestal, each and every time you hook up.  In this way, you will know what the marina is delivering.  I recommend you become meticulous about this.  If you are not receiving 240VAC – if you are receiving only 208VAC – you will have to make decisions about what to do next.  Do not expect the dock hands that help you tie up to know what they have. Some may, but I would assume many would not.  Frankly, even the marina manager may not know.

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DC Electricity On Boats

About This Article

This Article discusses DC Electricity concepts and terminology at an introductory level.  There are always discussions on boating bulletin boards relating to DC power systems on boats.  This article is intended to help those with little or no background or training in electrical systems to understand those discussions.  I have included the most important sub-topics related to 12V and 24V “low-voltage” DC power distribution systems encountered by typical cruising boat owners.

Electrical Safety

There is one, and only one, absolute when dealing with electricity.  VIRTUALLY ALL ELECTRICITY CAN BE DANGEROUS TO PROPERTY AND LIFE.  Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.   The large batteries and large banks of batteries found on boats can produce explosive gasses and store enough energy to easily start a large, fatal fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity!  WEAR HEARING PROTECTION when working in noisy environments, with running engines or other loud machinery.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right test equipment and tools for a job…
If you are not sure you know how to use the test equipment and tools you do have…
Well, then, LEAVE IT ALONE until you do!

USE INSULATED TOOLS when working around electricity, and especially around batteries.   Batteries contain enormous amounts of stored energy.  Accidental contact of a metal tool across the terminals of a battery is an emergency situation.  The tool can actually weld to the battery terminals and be both too hot to touch and impossible to remove without external mechanical force. Whenever working around a battery, pre-plan to have a two foot piece of 2”x4” readily available at hand.  If the worst should happen, use the wooden 2”x4” to knock the metal tool away from the battery terminals.  DO NOT TOUCH the tool; assume it will be far too hot to handle with bare hands!  Once this cascade of events has started, the only way to stop it is to break the tool free of the battery terminals.  Otherwise, the battery will get so hot it will melt and may start a fire.

Be very wary of unfamiliar, pungent odors.  Transformers, motors and most electrical and electronic devices that are in the process of failing often heat up and cause insulating or potting materials to give off strong, pungent odors. TURN OFF POWER and use your nose to track down the source.  Treat this as a true emergency.  If you can find the offending device before it bursts into flame, you’re way ahead of the game!  Turning off the power will usually allow the device to cool off.  Do not restart the device!  Excessive over-heating often causes secondary internal damage that you cannot see.

What Is DC Electricity?

DC voltages at their source are characterized by 1) a stable voltage amplitude of 2) unchanging polarity; i.e., the polarity of the voltage between the supply and return terminals never changes.  One battery terminal is considered “positive” and marked with a “+” sign, and one battery terminal is considered “negative” and marked with a “-” sign.  Terminals are either “positive” or “negative” with respect to each other, nit the external world.  The “positive” terminal is positive with respect to the “negative” terminal; the “negative” terminal is negative with respect to the “positive” terminal.  This distinction is important in using a voltmeter to measure voltages.  A DC voltmeter will provide both the amplitude of the voltage that’s present and the polarity of the conductors or components between which the meter is attached.   The amplitude of the voltage can vary somewhat over time, as over the period of time that a battery discharges, but the polarity of that voltage between battery terminals does not change.  This is the fundamental difference between AC and DC electricity, and that difference leads to all of the technical advantages and disadvantages the different electricity technologies offer to users.

Key Electrical Concepts and Terms

The following are some terms regularly used in listserv posts and widely encountered in discussions of electrical systems and circuits.  Boaters will do themselves a great favor by learning these terms and understanding the concepts these terms represent.

  1. Source – Point-of-origin of an electric current.  Typically for DC systems, a battery or bank of batteries.  Electrical sources are “balanced systems,” in that whatever current leaves must return to the source on a one-for-one basis.  If a return path is not available, current cannot flow and useful work cannot be performed.
  2. Load – The components within an electrical system that consume electrical energy to operate; ex: lights, heating elements, motors and electronics.
  3. Circuit – a network of conductors and components carrying electric current from the source to the load, distributing current throughout the load, and returning current to the source from the load.  Circuits are always closed loops that originate AND terminate at the power source.
  4. Supply (or “B+”) – the current-carrying conductor that transports electric current from the source to the load where power is consumed.  In “negative ground” DC systems as required on boats, often called “B+.”
  5. Return (or “B-”) – the current-carrying conductor that returns power from the load back to the source.  In negative-ground DC systems as required by ABYC on boats, often called “B-.”  Analogous the the “neutral” conductor in AC circuits.
  6. Voltage/Volt – the unit of quantification of “Electromotive Force” (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive force is measure across two points in a circuit.  (Volt, millivolts)
  7. Ampere/Amp – the unit of quantification of current flowing through a particular point in an electric circuit.  (Amps, milliamps)
  8. Current – the flow of electrons through a conductor, or the flow of ions through a liquid medium such as salt water; electric current is what performs “work,” i.e., fulfills the purpose of a circuit.  (Ampere; Amp)
  9. Resistance – physical property of all electrically conductive media that acts to retard or impede the flow of electrons through it.  All conductors have resistance. (Ohm)
  10. Conductor (lead, line, cable) – circuit device that transports electric currents.
  11. Ohm’s Law – Mathematical formula that describes the relationships between voltage (V), current (I), resistance (R) and power (P) in a circuit.
  12. Power – the quantification of the amount of “work” that electric current performs in its application.  In purely resistive applications, this will be light or heat.  In turning a motor, this will be the amount of electrical energy consumed in creating torque.  (Watt) (appropriate torque unit)
  13. Ground – a) a universal standard earth reference voltage of “0” volts; b) conversationally, the portion of an electrical circuit to which all other parts are referenced.
  14. Common – any interconnected portions of circuit to which many other parts of an electrical system are also connected.  If reference is specifically to “ground,” this term references a “return” return path shared by many separate portions of an electrical system.  Example: the  positive and return conductors to flybridge nav instruments may be supplied by a “common” B+ power feed conductor (red) and wired with a “common” B- return (yellow) conductor.  Analogies: “ground,” “B+,” “B-” “buss.”  Opposite: “home run.”
  15. Neutral – a non-ground, normally current-carrying return path for electric currents; customarily used in the context of AC circuits.  In DC applications, B- conductors are analogous to the AC neutral.
  16. Fault current – a current flow that follows an abnormal and unexpected path from its source to its return point.
  17. Short circuit – an electrical fault condition resulting from the unintentional connection of a source directly to a return circuit or earth ground.  This unintentional connection often results in the flow of extremely large fault currents. The electrical system should be designed in such a way that fault currents are automatically interrupted by circuit breakers of fuses.  This condition may not cause overload protection devices (circuit breaker) to disconnect the source of power in “ungrounded” systems..
  18. Chase – enclosed spaces in a building or a boat through which wires are run to achieve access to remote locations
  19. Raceway, Conduit, Spiral Wire Wrap, Split Wrap – varieties of supplemental physical enclosure intended to protect electrical conductors from accidental physical damage, excessive ambient temperatures and vibration.
  20. Switchgear – a generic term for all equipment housings in which fuses or circuit breakers and similar disconnecting or switching devices are mounted.  This term is used across the electrical power industry, from generating stations to transformer yards to neighborhood distribution yards to commercial and residential locations.
  21. ABYCAmerican Boat and Yacht Council, Annapolis, MD.  An organization that produces a comprehensive set of safety standards applicable to boats and boat manufacturers, the marine insurance industry, surveyors, attorneys involved in litigation and boat owners.
  22. NECNational Electric Code; United States electrical design standards for Power Generating and Distribution Systems, state, county and community code regulations  and the electrical construction industry.
  23. NFPANational Fire Protection Association; organization that creates and maintains the NEC.

DC Circuits

Fundamental Concept

The essential components of all electrical circuits are:

  1. a source of electrical energy,
  2. a conductor that transports electric current from the energy source to a load,
  3. an electrical load, where useful “work” results when an electric current flows, and
  4. a conductor that transports the electric current back from the load to the energy source.

By definition, an electrical “circuit” must contain all four of the above elements.  All electrical circuits (DC or AC) originate as a pair of electrical terminals that are connected to power-consuming load devices by conductors (wires) of one type or another.  Electric current flows through a circuit.  If a complete electrically-conductive loop is not available from “source” through “return,” an electrical current cannot flow.  Switches, fuses and/or circuit breakers are used to create an incomplete electrical path from the source to the load.

An electric current is the aggregate of millions of migrating electrons (and ions in liquid media).  In DC circuits, it is “convention” to think of the electric current flowing from positive to negative.  This “convention” Is a “working agreement” across all electrical standards bodies, trades and professions.  By mutual agreement, all electrical diagrams of DC circuits and electronics circuits are shown with symbols that assume current flows from positive to negative.  It is a fact of atomic physics that electrons carry a negative electrical charge, so migrate from a more negative place to a more positive place.  As in most “conventional agreements,” as long as the convention is agreed and understood, the pesky facts of atomic physics can be overlooked and left to scientists.

Circuit “Common” Reference(s)

The term “common” applies broadly to circuit elements that are shared among all of the broader network of electrical attachments in a installed electrical system.  The supply buss (“hot, B+”) and the negative return buss (“B-”) are examples of common circuit elements.

Virtually all DC systems encountered by the general public are low-voltage circuits, generally 12-volts, occasionally 24V or 32V.  Examples are 12-volt motor cycle, automobile, light truck, lawn tractor, residential emergency generator, snow thrower or all-terrain vehicle starting batteries, and similar yard and garden devices.  Other low-voltage battery-operated devices include fire/burglar alarms, Uninterrupted Power Supplies (UPS) for computers and data networks, hand-held spot lights, wireless telephone systems and a very wide variety of portable tools.

For applications in the automobile, truck, and outdoor equipment sectors, the return terminal of the battery is typically attached to the metal frame of the vehicle/equipment upon which the  battery is mounted.  The frame is the “common” return path for all sub-circuits.  Electrical components (starter motors, blowers, horns, light sockets, solenoids, sensors, gauges, electronics, etc) have internal electrical return connections that attach to the vehicle’s frame.  The electrical connection is created when the component is bolted to the chassis of the vehicle.  No discrete return conductor is needed because the metal vehicle chassis is the common electrical return path.  This approach simplifies wiring and mechanical design, reduces component design complexity, reduces material and labor cost, and eliminates wiring and connector materials and weight.  The metal frame of a vehicle is perhaps the most obvious place where the term “common” would describe a broadly-shared circuit component.

There are several factors that affect the preceding discussion as it applies to boats:

  1. most small and mid-sized pleasure craft are wired with 12-volt DC systems; 24-volt and 32-volt DC systems are sometimes used;
  2. some medium and large-sized boats have hybrid DC systems of mixed 12V, 24V and 32V systems;
  3. fiberglass (fiberglass reinforced plastic, FRP) does not conduct electricity, so fiberglass boat construction does not provide a functional “chassis,” or “vehicle frame,” return path; and,
  4. electric currents of even the smallest magnitude flowing in metal hulls, metal stringers and/or metal frame members lead to corrosion of the metals, and are always undesirable on boats.

Electrical appliances and utility attachments intended for marine DC applications are designed to have at least two wires; one for the supply of current that originates in the source (B+), and one for the explicit return of that current back into the source (B-).

Ground

In all of the preceding discussion, I’ve intentionally referred to the “electrical return path” using that specific term.  In ordinary conversation, the term “ground” is often used to describe the return path of a DC circuit.  This is a technical “liberty” of conversation, since DC return paths are often not actually connected to earth ground.  “Ground” in this context is a term of convenience and convention.  The return path from a low-voltage DC load to its source (B-) is not inherently at zero volts with respect to its surroundings.  A battery held in hand or sitting in a dock cart has two terminals, but neither is referenced to it’s surrounding environment.

Consider a bird sitting on a high-voltage overhead wire in a residential neighborhood.  The wire is at thousands of volts with respect to the earth, and so is the bird’s body and all of it’s little body parts.  But, the bird is safe, the tiny electric currents that make the bird’s heart beat still work, because there is no return path from the bird’s body to enable a disruptive external current to flow.  As soon as the bird flies off, the voltage is gone.  The bird’s body voltage changes, but the bird’s heart still beats normally, and the bird survives, completely unaffected for the contact with that man-made high voltage.

Consider a car, then, that is mounted on rubber tires.  Since rubber is a fairly good insulator, it would be possible for a DC voltage to exist between the earth and the frame of the car.  Normally small, this voltage can be thousands of volts.  Readers who have ever visited or lived in cold climates are undoubtedly familiar with the static shock that can happen when exiting a the car.  That static shock was a blast of high voltage DC caused by the transfer of accumulated charge from the vehicle, through the body, to the earth.  (Well, for purists, electrons flow from the earth, through the body, to the vehicle, but to the shockee, that detail is uninteresting.)  Static electricity and lightening are the same phenomenon, only on a much different scale!  The possibility of static shock is why every gasoline dispensary in the country instructs drivers to remove portable gas cans from cars and place them on the ground before filling them.  Grounding the container disburses any static charge.

It is technically non-trivial to create a reliable earth ground on a car.  Some readers may have seen ground straps dangling from trucks and some cars.  Used mostly on trucks, those straps are intended to protect toll takers and others who might come into contact with the vehicle from static shocks and to provide a safe path to ground static charges.  It is obviously difficult to create a reliable earth ground on a boat, impossible on an airplane.  Historically, earth grounding was not regarded as an important design goal for DC electric circuits.  And of course, experience with low voltage DC equipment generally bears out that assumption.  We get shocks from the build-up of static, but we don’t get shocks when we step off the garden tractor or use the snow blower.  Who among us has never disconnected a car battery when standing on the ground, and that was not a shocking experience.

What this all implies is that, even though the DC return circuit may not actually always be at the electrical potential of earth ground, the DC return circuit in all of our familiar yard equipment, cars and SUVs is referred to in ordinary discussion as the circuit’s  “ground”.  This use of the term “ground” refers to the functional return path ground, not a safety ground.

Safety Ground

As society gained experience with electricity in the early and mid-20th century, it became obvious that there had to be a way to ensure the return path is always at “earth ground” potential in order to  avoid the possibility of personal harm or property damage resulting from accidental contact with electric power.  A safety ground is not required for a circuit to operate correctly, but it does provide other compelling benefits.

Consider a fiberglass boat.  Aboard, there are many parallel DC sub-circuits.  Water pumps, space and nav lighting, nav and entertainment electronics, windlass, thruster, the propulsion engine, etc.  They are all at distances from one another, and the fiberglass frame of the boat is non-conductive.  A safety ground in a DC system (if present) interconnects the external frames and metal cases of equipment, appliances and utility attachments (light switches, outlets, motors, electrical equipment, radios, etc.) to a known point of common potential.  That common point is always the negative terminal of the battery, and under some specific conditions, the water in which the boat floats or the earth itself.  A safety ground is separate from the functional return circuit, and always involves the installation of it’s own individual electrical conductors.

In service, a “safety ground” is never intended to carry current in normal operation.  However, in a circuit containing an electrical fault condition, a safety ground is intended to prevent a personal shock hazard or mitigate property damage risk by ensuring the electrical potential is at earth ground potential.  It is the “safety ground” that provides an emergency path that allows a circuit breaker to function and disconnect power.

Consider, for example, a bow thruster or an anchor windlass.  We would expect to have a battery positive connection to the positive (B+) terminal of the device’s motor solenoid, and a battery negative connection to the negative (B-) terminal of the device’s motor solenoid.  The motor would then be expected to operate correctly with just these two battery connections.   If we also had a separate conductor from the mounting frame of the device to the vessel’s bonding system, that would be considered the “safety ground.”  The thruster would run just fine without the safety ground, but the device could malfunction and place the frame at some non-zero electrical potential.

Vessel Design – “Grounded” vs “Ungrounded”

Designers of DC electrical distribution systems refer to them as either “grounded” or “ungrounded” systems.  The terms “grounded” and “ungrounded” refer to the presence or absence of a safety ground, not the functional return circuit.  A return path of electrons to their source is always required, but that return path is not always referenced to anything else!

There is valid debate among experts as to whether 12-volt, 24-volt and 32-volt boat DC systems can be of the “ungrounded” design or should be of the “grounded” design.  Today, DC grounded systems are not common.  However, new and emerging vessel propulsion systems containing large-horsepower (hP) diesel-driven DC generators and large-horsepower DC motors (systems analogous to diesel-electric train locomotives) are definitely high voltage applications (often between 600VDC and 1000VDC).  Faced with the emerging presence of true medium and high-voltage DC equipment on pleasure craft, this safety ground design choice is now specifically being re-evaluated in the American Boat and Yacht Council’s (ABYC) Electrical Technical Committee.  We await that outcome.

It is to the advantage of boat buyers and all boat owners to understand the low-voltage DC electrical distribution system.  It’s also an obligation of the buyer/owner to understand whether or not a medium or high voltage DC system is also present.  In the majority of fiberglass-hulled boats, it would be unusual to have a separate DC safety grounding circuit installed.  On some boats, nevertheless, one could encounter one of several possibilities.  The electrical system installation on any individual boat depends on:

  1. the prevailing electrical construction standards at the time of OEM fabrication, often related to prevailing standards of the international geography where the boat was built,
  2. how many people may have added to, or otherwise modified, the system over time, and
  3. the electrical skills those individuals who have performed electrical work in the highly specialized marine environment.

The possibilities aboard a vessel include:

  1. no low-voltage DC safety ground at all (most typical today),
  2. partial DC safety grounding on some parts of the system (not recommended; considered technically inadequate), and
  3. full DC safety grounding, vessel-wide.

The ABYC does not require that low-voltage DC distribution systems have a safety ground, but it does make “recommendations” as to how “grounded” and “ungrounded” systems must be interconnected with the vessel’s bonding system.

Polarity  – “Negative Ground” vs “Positive Ground”

Earlier, I pointed out that a battery held in hand or sitting in a dock cart has two terminals, but neither is necessarily referenced to ground.  All that can be said is there is a fixed voltage between the two battery terminals.  Whichever battery terminal is connected to the vehicle frame determines the polarity reference for that DC system.  If the negative battery terminal is connected to the vehicle chassis, the system is considered to be a “negative ground” system.  If the positive terminal is connected to the vehicle frame, the system is considered to be a “positive ground” system.  With the emergence of solid state electronics and economic pressure to reduce manufacturing cost by sharing components across brands, models and manufacturers, the modern automobile industry world-wide (at least since the 1980s) has standardized around negative ground systems.

The ABYC-approved, and by far the most common, DC systems found on pleasure craft in North America are “negative-ground” systems.  On a boat with other than negative-ground DC distribution system, the panels throughout the boat should be clearly marked to identify the manner of connection.  If there is any doubt, always use a voltmeter to confirm the configuration before disconnecting or otherwise making modifications to the system.

Electrical System Topology

Electrical System Schema:

The schema of Sanctuary’s vessel-wide electrical system contains three major divisions.  This diagram is specific to Sanctuary, showing two 30A shore power connections and a fully-integrated but modestly sized inverter/charger.  That said, the overall model generalizes very well to larger electrical systems based on voltage, inlet, inverter/charger and load capacities and configurations.

  1. AC electrical system division of the vessel includes:
      • 120V Shore Power inlet connections
      • AC Generator (Genset)
      • ABYC-compliant Generator Transfer Switch
      • AC Branch Circuit Distribution Panel(s) – (NewMar – House loads; Weems & Plathe – heat pumps, raw water circulator)
      • Galvanic Isolator
  2. DC electrical system division of the vessel includes:
      • Battery Bank
      • Propulsion engine alternator
      • DC Branch Circuit Distribution Panel(s)
      • Individual component attachments (Thrusters, Windlass, Autopilot, Entertainment, Inverter/charger, etc.)
  3. Interface, or Bridging, or Power Conversion division of the vessel includes:
      • Magnum MS2012 Pure Sine Wave Inverter/Charger

General Topology of the Vessel Electrical System:

topography

An Adobe Portable Document Facility (.pdf) version of this drawing is available by clicking this link: 20161019_electrical_system_topology.

Bonding System Design and Evaluation

My previous post (Corrosion Article) discussed corrosion of underwater metals caused by various stray electric currents in the water.  In that post, I made passing reference to “bonding,” “bonding conductors,” and to underwater metals “being bonded together.”  This article looks specifically at the bonding system of a boat.  The objective is to provide a basic understanding of why bonding is installed, what it does, and consider the maintenance needs of the bonding system.

As boaters, we are constantly involved in discussions of the design, equipment, materials, techniques and components of the AC and DC divisions of a boat’s electrical system. When those systems fail, there are usually symptoms, anxieties and inconveniences that boaters notice. Although Internet boating discussion lists are filled with electrical topics, only rarely does one see discussion related to a boat’s “bonding system.”

As in other electrical technical areas, “bonding” is an area where there is a body of common concepts and terminology that apply across a wide range of AC and DC situations. Just as in the “Corrosion” topic, the concepts of bonding are consistently the same, but an understanding of context is essential to avoiding confusion.  Experienced electrical practitioners often take shortcuts with context. For the layman, the only way to get past that is to invest some time in understanding the concepts. After that, understanding context gets easier very quickly.

The terms “grounding” and “bonding” are often used interchangeably, but in fact, they are different. Following are definitions with which most experts would agree:

Ground” is the single-point of electrical connection between an electrical sub-system (like a boat) and the physical earth. This connection is made for the purposes of:

  1. providing a lightning discharge path,
  2. providing a path to bleed off static charge,
  3. sub-system voltage stabilization, and
  4. reducing RF interference.

Bonding” is an electrical connection (usually a network of electrical connections) which electrically interconnect metallic housings and device enclosure components. Bonding:

  1. provides a low-resistance path for ground-fault currents to ensure circuit protection devices (circuit breakers) trip,
  2. prevents dangerous “touch-voltages” from appearing on exposed metal surfaces, and
  3. provides a path for galvanic currents and AC and DC stray currents.

Figure 1 is a simplified topology overview of the three major divisions (AC division, DC division and Bonding division) of the electrical system of a typical boat. It is representative of the great majority of US-manufactured boats. This topology view is consistent with the “model” electrical system upon which the principal ABYC Electrical Standard, E-11, is based (“AC and DC Electrical Systems on Boats,” July, 2015, Figure 10).

The ABYC E-11 standard treats a boat’s DC System as the “central-most” division of the electrical system of the boat, to which all other divisions are attached in a peer-to-peer relationship. This seems a reasonable assumption, since AC systems and bonding systems are neither required nor essential on a boat, but the DC system is always needed for engine starting and the operation of bilge pumps, navigation lighting and (usually) sound-signaling device requirements.

Total_System

Figure 1: The Bonding System on a Typical Fiberglass (FRP) Cruiser.

All of the conductors shown in green in Figure 1 are part of the boat’s “bonding system,” or “bonding network.” That entire network of conductors works together. In typical dockside conversation, the “bonding system” is often thought of as limited to the wiring shown on the right-had side of Figure 1. The usual term applied to the AC portion of the bonding system is the “AC safety ground.” Note, however, that the AC safety ground is a part of the overall bonding network of the boat.

In normal operation, all bonding systems are “silent” and “invisible.” When “everything is right,” the bonding system does nothing, and “everything works fine.” Bonding networks are so quiet and invisible that a boat owner might never know if a fault had appeared.

In fact, the primary purpose of the bonding system is to spring into action to protect us when an electrical fault does occur in either the AC or DC system. The only “normally active” purpose of the bonding system is to control corrosion due to DC galvanic currents.

Due to component reliability, the mathematical probability, confirmed by life experience, is that electrical faults are relatively infrequent. Given that the bonding system comes into play only when there is a fault, it probably won’t actually be needed very often. If the bonding system does have a defect, unless there is another fault there will be no failure symptom or danger to people or pets. Yes, there may be an increased rate of corrosion, often interpreted as “electrical issues in the basin and nothing to worry about.” These are “handled” as a routine maintenance item, but the underlying cause is often not corrected. The bonding system adds complexity to the boat, but can save many headaches, much expense and even heartache for the boat owner if it is intact when needed. Some bonding system faults can create dangerous situations leading to fire, electric shock, loss of property and in the ectreme, loss of life.

The heart of the DC division of the boat electrical system is the battery/battery bank, including all B+ and B- wiring and all subordinate DC device attachment wiring. “B+” is the term for the DC positive feed (+12V, +24V) that originates at the positive post of the boat’s battery. “B-” is the term for the DC negative conductor that returns DC power to the negative post of the battery. In the common lexicon of conversation, the DC return circuit is often referenced as its “ground” conductor. However, the B- conductor in the DC system carries DC current back to the battery, so it is more properly analogous to the “neutral” conductor of the AC division.

Bonding circuits are intended to carry only galvanic and fault currents; never currents that power equipment or attachments. To avoid undesirable voltage drops in the bonding system, and problems with accelerated electrolytic corrosion, no B- connections should ever be made to any part of the bonding system. Such connections are analogous to a “code violation.”

ABYC E-11, Figure 10, shows the “DC Main Negative Buss” as the central collection point for all DC B- return circuits, as well as for the “AC Safety Ground” and the bonding network connections. The boat’s AC Safety Ground and the various branches of the DC bonding system are all connected together at one place, and at one place ONLY: the “DC Main Negative Buss.”

Neither ABYC nor NMMA “require” the installation of DC bonding systems. Bonding systems are “optional.” However, ABYC E-11 does specify requirements for the bonding system if one is installed. Among US boat manufacturers, bonding systems are the “normal” manufacturing practice.

The primary purposes of the bonding system are to:

  1. hold exposed metal parts at to “touch potential” that is safe for people and pets;
  2. provide a low resistance path for fault currents to trip “circuit breakers;”
  3. provide a single point-of-access to protect multiple structural metals of the boat from corrosion, via a sacrificial anode (zinc);
  4. provide a path for certain DC stray currents to safely exit the boat via the AC shore power safety ground;
  5. disperse static electricity in high winds and from nearby electrical storms, and
  6. reduce (attenuate) spurious RF electrical “noise” created by on-board equipment (battery chargers, inverters).

Many of the conductors of a “bonding system” are installed in the very hostile environment of the boat’s bilge. The various metal objects tied to the bonding system include:

  1. thruhulls, seachests, sea strainers and packing glands,
  2. rudder “stem iron,” rudders, rudder “shoes” (skegs), tillers and miscellaneous metal support structures of the steering system,
  3. various steering system components (quadrant, cables, hydraulic lines, hydraulic pumps),
  4. trim tabs and thruster systems,
  5. exhaust system fittings and ports,
  6. radio counterpoise and static dissipation “ground plates,”
  7. fuel tanks, fuel filling ports and tank vents,
  8. potable water and black water tank access and vent ports,
  9. generator, battery charger and inverter chassis frames,
  10. solar panel and wind generator frames,
  11. handrail and bridge enclosure frames,
  12. heat pump and circulator pump frames,
  13. stove and water heater frames,
  14. refrigeration (compressor) frames,
  15. etc, etc, etc…

In short, lots ‘o stuff.

Figure 2 shows the hull penetrations on a typical trawler (Sanctuary) built with individual thruhulls (without a seachest).

Figure 2: Typical Hull Penetrations on a Boat with Thruhulls and Without a Seachest

The complete collection of all of these metal components are “bonded” – connected together into a single electrical network – as shown in Figure 3.

Figure 3: Typical Bonding System

Figure 3 is only one example of the construction of a bonding system. Other configurations are acceptable. Take particular note of the large gauge conductor shown in orange. That conductor is the “backbone” of the DC portion of the bonding system. That backbone conductor runs the length of the hull. To the backbone are attached all of the green stranded wire pigtails connecting the metal structures of the boat to the backbone. Also note the transom zinc, which provides primary galvanic protection to all of the metals connected to the bonding system. When the boat is at anchor, away from shore power, it is the transom zinc that is the “ground” connection point. That is, the single point of electrical attachment to the earth, the primary dispersal point for static electricity and lightening and the electrical connection that establishes the “touch potential” for people and pets for the entire electrical system of the boat.

It would not be unusual if a boat’s owner did not know when the bonding network was last tested. It may have been quite some time; perhaps, never. It is possible that weakness(es) are present in the bonding system. I suggest testing of the bonding system should be done every three to five years.

Most if us have measured the terminal voltage of flashlight batteries many times. We have probably all measured our boat’s 12V (or 24V) lead/acid batteries. Figure 4 reminds us of the very simple task of measuring the terminal voltage of a “AA” battery:

measure_battery

Figure 4: Measuring the Terminal Voltage of a Battery

In this “typical” battery, a galvanic cell, there are two “half-cells” (copper and zinc) located in an electrolyte. Since the battery is always seen as a packaged unit, the term “half-cell” is not commonly used except by engineers and battery manufacturers. The terminal voltage is measured with a digital voltmeter. When a load is connected across the battery terminals, current flows.

Key concept: batteries are used to provide the voltage needed for circuits.  With batteries, their intended use means there should be a voltage between the positive and negative terminals.  A direct short circuit across a battery is never desirable, as it will dramatically accelerate the rate at which the battery becomes exhausted.  Inside a short circuited battery, the halfcells will become wasted (corroded) at an extremely fast rate, accompanied by the generation of heat and gasses.  However, in the case of the “accidental” battery created by the electrochemistry of dissimilar metals in seawater, the whole point of the bonding system is to create an electrical short circuit across the various exposed terminals of that “battery.”  Bonding creates a path for electrochemical galvanic currents to circulate.  Bonding holds all of the metal surfaces at the same, safe touch voltage, but in so doing, bonding also ensures the presence of the conditions needed for corrosion to occur.  That is the reason for the presence of the transom zinc in the bonding network.  The transom zinc is the sacrificial anode that protects all of the important and more noble metals attached to the bonding backbone from corrosion.

For measuring and troubleshooting the bonding system of a boat, a reference “half-cell” is used. The reference cell is external to the bonding system.  The reference cell behaves in a known and predictable way when submerged in sea water. The reference cell becomes one of the halves of a “battery.” The metals attached to the bonding network of the boat become the other half-cell. In use, the reference half-cell is immersed in seawater outside the hull of the boat, and that seawater is the electrolyte of the “battery.”

A Silver/Silver Chloride half-cell is the best reference cell with sea water (chemical symbol: Ag/AgCl) because it has known and stabile behavior characteristics. That is, the voltage that other metals will produce against a silver/silver chloride half cell are very consistent across a wide range of temperature and electrolyte salinity.

Conceptually, measuring between the Ag/AgCl half-cell and the bonding network of the boat is the same as measuring the between the terminals of a conventional battery. The bonding system and the half-cell, immersed in sea water, become the battery being tested. The DVM measures the terminal voltage of that battery.

Figure 5 shows the measurement configuration described above:

measure_hull

Figure 5: Measuring the Bonding System with a Ag/AgCl Half-Cell

As a boat owner, there are two ways to proceed with the testing of the bonding system. One is to hire an ABYC-Certified Corrosion Specialist. This analysis is a form of survey, although not all surveyors offer it as a service. Two is for owners to do it themselves.  In the DIY case, one must obtain an Ag/AgCl half-cell, available from http://www.boatzincs.com and other Internet sources at a cost in the range of $140 – $150.

DIYers will begin their testing by connecting the Ag/AgCl half-cell to the negative terminal of the DVM. Then lower the Ag/AgCl half-cell over the side into the water near the hull, to about the level of the boat’s running gear. The half-cell should not rest on the sea bed. The guiding principle here is, if the bonding system is fully intact and functional, all metals connected to the bonding system are expected to be at the same voltage. Probing any of the bonded metals with the DVM should produce the same voltage reading. If different voltages are noted, something is not right, and corrective action is advised.

The bonding system of a boat – whether connected to shore power or not – should produce a reading on the DVM of between -400mV and -700mV. Knowing that the bonding system has all of its metal structures tied together, we therefore know all of the readings must be found at the same voltage.

To evaluate the integrity of the bonding system, start anywhere that’s convenient and probe each of the various metal objects found all over the boat; that is, all the stuff previously mentioned (thruhulls, packing glands, sea chests, rudder posts and rudders, steering system components, exhaust fittings, main engine/transmission, Generator frame(s), battery charger/inverter chassis frames, solar panel and wind generator frames, handrail and enclosure frames, heat pump unit chassis frames, fuel tanks, fuel filling ports and tank vents, potable water tanks, thruster systems, black water tank, etc, etc, etc). The voltage measured by the DVM should be the same as seen at the shore power connection everywhere. If it is not, something is “wrong!”

The last two steps in this analysis are to discover the cause of any inconsistent voltage reading, and make corrections. Some symptoms one might encounter include:

Symptom Possible Cause
Wide variation of voltages between different metal objects.
  1. Boat is not fit with a DC bonding network;
  2. Damage or corrosion to connections within the bonding system.
Most metal objects have consistent voltages except for one or two isolated objects, “here and there.” Loose, corroded, broken or missing bonding connections to the affected metal object(s).
A collection of several metal objects measure one voltage, but that entire collection is different from the baseline voltage. Broken bonding buss somewhere along the length of the backbone.
The baseline voltage is grossly different than expected (-400mV to -700mV).
  1. Loose, corroded, broken or missing connections to the transom zinc or the shore power ground. Disconnect from shore power, looking for changes and to check the transom zinc by itself;
  2. Overly wasted transom zinc;
  3. Missing shore power ground connection;
  4. B- connection to the bonding system made in error;
  5. Stray DC electrolysis current in the bonding system.
No reading occurs when the metal object is probed. Bonding connections absent.

(Note: this will only happen with metal objects above the waterline and not in contact with the water.)

Metal Corrosion and Zinc Wasting

INTRODUCTION:

On the long list of complex technical topics that boat owners face, corrosion of underwater metals is one of the most complicated, potentially most expensive and least well understood.  While it is not possible to ease the complexity or terminology of the topic, I can at least describe several related “stray current” metal corrosion phenomena in this one place.

Some readers may feel this topic is “beyond their pay grade.”  Like it or not, we all have a stake in understanding the basics.   At some time in boat ownership, most owners will face one or more corrosion issues.  Even for those for whom the topic is both uninteresting and obscure, all boaters should know how these phenomena are similar and how they are different.  Some familiarity will allow the affected owner to hire the right expert, understand remediation recommendations, and possibly avoid problems in the first place.

AC and DC “stray” electric currents flow in the water.  Because these currents flow outside their normal electrical conductors and devices, they are referred to as “stray currents.”  Worst case, all types of fault currents can be present at the same time.  Boaters should consider all electrical currents that flow in the water as a bad thing.

FUNDAMENTAL CONCEPT OF CORROSION:

The basic concept in all corrosion is always the same: there is a voltage difference between two or more different metals, or alloys of metal, which are a) connected together electrically and b) immersed in an electrically active liquid.

The three major “stray currents” flowing in water (or in the earth’s crust) are:

  1. AC “ground fault” currents, resulting mostly from wiring errors aboard boats and occasionally from inadequate equipment design, incorrect equipment selection or AC appliance/equipment malfunction,
  2. DC “Galvanic” currents, resulting from the natural behavior of dissimilar metals in mineral-containing ground water, fresh surface water or sea water, and
  3. DC “Electrolysis” currents, a DC “ground fault” current, resulting from wiring errors, equipment faults, and improper equipment use.

While the basic electro-chemical processes and terminology of corrosion are always the same, the cause is context-specific.  Understanding the context (AC fault current, DC galvanic current, or DC Electrolysis current) is essential to avoiding confusion caused by the shared terminology.

“Electrolysis,” or “Electrolytic corrosion,” is  frequently confused with, but very different from, “Galvanic corrosion.”  To repeat, the concepts and terminology are shared and common to both phenomena; it is the “cause and origin” of the driving voltage that is different.  

No matter the terminology, corrosion currents are a silent attack on every boat, and can cost many hundreds or thousands of dollars for those who don’t mount an appropriate and effective defense.

AC FAULT CURRENTS:

AC fault currents flowing in the water are often dangerous to people, pets and wildlife.

Worst case, AC fault currents can lead to death by “Electric Shock Drowning:” (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/electric-shock-drowning/).  Children and pets must never swim in a marina’s basin.  Boat owners and professional divers performing in-water maintenance on boats must be alert to the causes and consequences of AC electric currents in the water.

In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting.   There is a great deal of technical understanding about environmental AC ground fault currents that comes from the utility and transportation industries (power transmission, buried utilities, pipeline and railroad).   The 60 Hz AC power found across North America changes polarity 120 times per second.   Whatever molecular metal material that might be removed from a metal in one half-cycle is re-deposited in the second-half cycle.  (Cit: “DC Currents in the Bilge – Not AC – Is the Culprit When Metal Fittings Corrode,” Robert Loeser, Seaworthy Magazine, October, 1996).   AC fault currents must be very large before metal corrosion results.   The AC voltages found around pleasure craft docks (less than 600V) DO NOT cause zinc wasting.

Aluminum can be an exception.  Aluminum can be damaged by AC stray currents IF the density of the fault current is greater than 40 Amps per square meter of aluminum surface area (40A/M2).  What that means in English is: a relatively high AC fault current in the water will cause erosion to a relatively small chunk of underwater aluminum.  This combination would be unusual, but not impossible, in pleasure craft marinas.  One m2 is equal to about 10.75 ft2, so it would only take 3.7 amps of AC stray current to cause corrosion damage to a 1 ft2 aluminum part.  This is not an extraordinary leakage current, but 1 ft2 is a small piece of aluminum.  So trim tabs and outdrives may be “relatively” “safe” at levels that would waste aluminum anodes installed for galvanic protection.  On boats without other aluminum parts, aluminum anodes can waste quite rapidly in proportion to the size of a moderate in-water AC fault current.

Readers can find information on testing for AC ground and leakage fault currents in layman’s language on this website, here:  https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/.   The reference article helps owners bring their boat into compatibility with National Electric Code (NEC) standards that require ground fault sensors on docks, and it also dovetails well with identifying and eliminating corrosion issues.

Be aware that some in-water AC stray currents can originate from sources on land.  In that case, the fault current will flow on the green AC safety ground wire (a component of the boat’s bonding system), originating in the basin water and flowing back into the shore power infrastructure.  This situation is not caused by a problem on the boat, and in general, is not something a boat owner can fix.  Always report this finding to marina management.

WHAT THE BOAT OWNER CAN DO: AC FAULT CURRENTS:

  1. Assess the boat for AC ground fault and leakage fault conditions.  (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/)
  2. Correct all issues in order to establish a defect-free starting-point baseline.
  3. Consider installing Equipment Leakage Current Interrupter (ELCI) sensors on boat shore power AC service circuits.  (ref: ABYC E-11, 11.11.1 and  https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/elci-primer/)
  4. Where automatic ELCI sensors are not installed, perform frequent manual monitoring of AC shore power cords with a decent-quality clamp-on Ammeter.  (ref: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/ac-safety-tests-for-boats/)
  5. Correct any newly discovered issues as soon as they present themselves.

DC GALVANIC CURRENT TERMS AND CONCEPTS:

  1. DC galvanic currents are associated with small voltage potentials that are a naturally-occurring characteristic of all metals.  The specific voltage is determined by the atomic structure of the individual metal (or metal alloy).
  2. Different metals have different naturally-occurring electro-potentials.
  3. A “Galvanic Series” is a list of metals sorted by their naturally-occurring characteristic electro-potentials.  Different “references” can be used for ordering a “galvanic series.”  The best reference for salt water is a silver/silver chloride cell.
  4. A “galvanic couple” is any combination of two or more dissimilar metals or metal alloys connected together electrically and immersed in an electrolyte.
  5. An “electrolyte” is an electrically conductive liquid (generally) medium.
  6. Dry Corrosion” is the direct attack on a metal by dry gasses (air, oxygen) through chemical reactions which result in surface oxidation.
  7. Wet Corrosion” is the direct attack on a metal by an aqueous solution (strong or dilute, acidic or alkaline) through electro-chemical reaction.  Moisture and oxygen can act by themselves.

DC GALVANIC CURRENTS:

The underwater metal alloys on a boat together with the minerals in the surface water in which the boat is floating create the elements of a “galvanic cell” (a battery).  Galvanic currents will always be generated when a boat with dis-similar metals occupies water containing dissolved minerals.  A zinc/copper galvanic couple (common “dry cell” flashlight battery) is a “galvanic cell.”  A “lead/acid” automotive or boat battery (wet cells, AGM or Gel) are examples of “galvanic cells.”

When the metals making up the galvanic cell (battery) are actually the underwater component parts of a boat (bronze, aluminum, stainless steel), the naturally-occurring galvanic currents result in corrosion of some of the underwater metal.   The mineral concentration of sodium, calcium and magnesium salts and many others in the surface water affect the speed at which galvanic corrosion proceeds.

The flow of electrons in a DC galvanic current is always from a more active metal (anode) to a less active metal (cathode) on a Galvanic Series.   All environmental surface water, whether fresh or salt, acts as an electrolyte.  Salt water carries more mineral ions than fresh water, so is more “efficient.”

Galvanic corrosion is a slow process that occurs over many months.  Since it’s the anode in a galvanic cell that dissolves, the point of avoidance/remediation is to artificially force the metal(s) to be protected relatively more cathodic compared to a sacrificial metal present in the electrolyte (water).  This is done by adding a “sacrificial anode” made of a very active metal (aluminum, magnesium, zinc) to the mix of less active but more valuable underwater metals on a boat.

Perhaps a ”before” and ”after” view:

unprotected_couple

Figure 1 shows a ”before view” of a galvanic couple lying in seawater.  The stainless steel (SS) is the anodic alloy, so it erodes due to the natural galvanic voltage between it and it’s cathodic bronze couple-mate.

protected_couple

Figure 2 shows an ”after view” of the same galvanic couple with the addition of a sacrificial zinc anode.  The zinc forces the SS relatively  more cathodic (relative to the zinc), so the SS part is now protected from corrosion.  The zinc is the most active (anodic) metal in this new couple.  By corroding, the zinc acts to protect all of the more “valuable” metals.

Ongoing zinc maintenance is required to provide continuing protection of the more important components of the couple.  Additional valuable corrosion control techniques include the installation of galvanic isolation devices in shore power ground conductors, cable TV coax ground sheathes, and the ground conductors of (now pretty much obsolete) wired telephone and wired Ethernet connections.

ENGINE COOLING SYSTEMS AND GALVANIC CORROSION:

The propulsion and genset drive engines on most cruising-sized pleasure craft are fit with two-stage engine cooling systems.  In diesel cooling systems, a coolant (“fresh water”) circulates through the engine block, heads, oil-cooler, turbo-charger, and intercooler.  A heat exchanger transfers waste heat from the fresh water coolant to environmental raw water, where it is eliminated via the raw water exhaust.  Commonly, a second heat exchanger transfers waste heat from transmission fluid into exhausted raw water.

Electro-chemically, the raw water passing through the heat exchanger is an electrolyte.   Heat exchangers contain several different alloys of copper and nickel.  The alloys used in heat exchangers are designed to have galvanic voltage potentials that are close to one another on the salt-water galvanic series.  That greatly slows, but does not stop, the galvanic corrosion which occurs within heat exchangers.  The dissimilar metals of the heat exchanger act as the galvanic couple and the raw water is the electrolyte.

If galvanic corrosion in heat exchangers is allowed to continue uninterrupted, pinpoint leaks will develop in the shell or tubes of the exchanger.  Similarly, pinpoint leaks can develop in raw-water cooled oil coolers, transmission coolers and intercoolers.   The result over time is damage to expensive heat exchangers, as well as the possibility of secondary damage to the engine itself.   Boat owners  must be aware that there are zincs located within the raw water channels in engines and heat exchangers.

Boats with wood, steel and aluminum hulls require special anti-corrosion techniques.  Many sacrificial anodes are required to protect the surface area of metal hulls.  Too many anodes can cause paint to peel from a metal hull, and cause damage to the woods of a wooden hull.  Alternatively, systems such as Electro-Guard (http://electro-guard.com/index.html) apply a voltage to a metal hull.   These “Impressed Current Cathodic Protection” (ICCP) systems protect the hull plates and welded joints from galvanic attack by making the hull cathodic to its surrounding environment.   This is one of many areas that are “different” for owners of metal-hulled boats vs hulls of fiberglass reinforced plastic (FRP).

SINGLE-METAL GALVANIC CORROSION:

SS, bronze, brass and galvanized steel are metallic alloys that contain several elemental  metals within their compounding mix.   Dissimilar metals within the alloy can experience galvanic corrosion.  “Single metal” corrosion results in micro-fractures in the material’s structure, and often results in surface pitting.  The process can proceed to structural failure.

Anodic and cathodic areas form on the surface of alloys due to surface imperfections in the alloy mix, lack of oxygen and/or other environmental factors.  The anodic areas in the matrix give up electron(s).  The ions left behind form into the visible hydroxyl oxidation residue that is shed.  Corrosion currents flow at the expense of the anodic metal of the circuit, which corrodes continuously.

SS shaft logs and propeller shafts, SS rudders and rudder posts, SS fasteners that attach swim platform brackets to an FRP hull, SS keel bolts, SS exhaust port fasteners, etc, etc, are all candidates for a form of single metal galvanic corrosion called “crevice corrosion.”

In brass that contains more than 15% zinc, like the manganese bronze alloy often used in propellors,  unprotected fittings can undergo a single metal galvanic corrosion process called “dezincafication.”  Zinc within the brass alloy erodes away, leaving behind a weak matrix of copper and small percentages of other metals (such as nickel, chromium, manganese) of the original casting.  What’s left is structurally weak and can fail catastrophically.  “Dezincification” leaves a characteristic “pinkish” color to what once had a golden bronze color; particularly so in broken, exposed areas of a part.

In stainless steel, this process is called “CREVICE CORROSION.”  In aluminum, the analogous process is called “POULTICE CORROSION.”  When stale water lies against stainless steel for long periods or time, the water looses it’s content of dissolved oxygen.  Oxygen-depleted water in prolonged contact with stainless steel promotes crevice corrosion, leading to possible structural failure in stainless steel parts and fittings.  Similarly, water that lies in contact with aluminum for long periods of time promotes poultice corrosion.  Poultice corrosion can result in pinpoint leaks in aluminum fuel tanks.

For thruhulls especially, boaters should use fittings of bronze or Marelon; BRASS FITTINGS SHOULD NEVER BE USED UNDERWATER.

For those interested, I have more details on Galvanic Corrosion and the Galvanic Series for salt water on this website, here: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-dc-topics/zincs-and-galvanic-corrosion/.

BoatUS has a good article on electrochemical corrosion on their website, here: http://www.boatus.com/boattech/articles/marine-corrosion.asp.

David Pascoe has a good article on electro-chemical corrosion on his website, here: http://www.yachtsurvey.com/corrosion.htm.

WHAT THE BOAT OWNER CAN DO: DC GALVANIC CURRENTS:

  1. Install a complete bonding system if one is not currently present.
  2. Install zincs to protect bonded underwater metals.
  3. Perform routine maintenance of zincs on underwater metals: propellor shaft, rudder, and other underwater metal structures.
  4. Maintain the “master” zinc that protects the boat’s bonding system.
  5. Maintain zincs protecting engine and transmission cooling system components.
  6. Use deck fill screw-on covers that are galvanically compatible with under-deck fittings to avoid galvanic corrosion and hidden fuel leaks.
  7. Install an appropriately rated Galvanic Isolator in the shore power safety ground if one is not already present.
  8. Install galvanic isolators to telephone, Ethernet and TV Cable feeds that come onto a boat from shore.

DC ELECTROLYSIS CURRENTS:

The source of the voltage that drives the process is what distinguishes a “Galvanic current” from “Electrolysis,” or an “Electrolytic” current.  Recall that galvanic voltages are a function of the natural atomic electro-potential of the metals of a galvanic couple.  The voltages which cause electrolysis are man-made, not naturally-occurring.  The voltages that drive electrolytic corrosion are often significantly larger than galvanic voltages, and the destructive impact of a DC fault causing electrolytic damage is much faster and more aggressive than galvanic currents.

In Figure 3, the metallic actors (SS and bronze alloys) are the same as shown in Figure 1.  In this case, the elemental voltage polarity of the couple has been reversed by the application of an outside source of DC voltage.  This is a DC fault scenario.  The bronze thruhull in this example will disintegrate, freely giving up it’s copper content into the surrounding sea water.

electrolysis

Figure 3: A DC Fault Injecting Power to the Bonding Buss Resulting in Corrosion of Important Underwater Metal Alloys

Electrolysis voltages originate with an externally- supplied DC source; i.e., a battery or its equivalent.  Causes can be a wiring error, chafed/frayed DC conductor, defect or age-deteriorated insulation on a bilge pump B+ wire lying in bilge water, a defect in a DC power supply or a DC generator, a failed piece of DC equipment or misapplication of use of DC equipment.   A common wiring error that can lead to electrolysis currents results from incorrectly wiring the neutral return circuit of a DC device to the boat’s bonding system.  NEVER USE THE DC BONDING SYSTEM FOR THE ELECTRICAL RETURN PATH FOR DC CIRCUITS.

DC electrolysis currents are equivalent to the industrial process called “electroplating.”   In a marina, failed DC equipment can deliver a DC voltage into the basin water.  On a boat, wiring error or a failed piece of equipment can apply a DC fault voltage to the boat’s ground buss.  Electrolysis current flows IN ONE DIRECTION through the ground path and into the surrounding basin water.  The anode literally dissolves.  The fault can be on the same boat as the failed equipment, on a neighboring boat or in nearby land based equipment.  Or it can be, simply, in between a source point and a return point.  The fault can be located in shore-side infrastructure wiring, or it can be because of misuse of equipment by a contractor, such as a welder or a DC motor on a marine railway or travel lift.

It is a law of physics that electric current always seeks the path of least-resistance back to their source.  Scenario: imagine three adjacent slips on a dock.  In slip #1 is a boat with a fault and dumping a DC electrolysis current into the basin water.  Slip #2 is empty.  In slip #3 is a boat providing a path to ground for the fault current via it’s shore power cord.  So far, only boat #1, the boat with the fault, has a corrosion problem.

Now a transient boat comes into slip #2.  The fault current previously passed through slip #2 on the way to ground, but when the transient arrived, that boat’s protective electrical system (Bonding System) becomes inserted into the path of the fault current.  The transient’s bonding system has a lower resistance than the surrounding basin water.  The fault current passes into the transient via one or more underwater metals, passes on through the transient’s bonding system, and exits back into the basin “on the other side” of the transient.  The fitting(s) where the current exits the transient will corrode.  That fault current is now causing damage to boat #1 and boat #2.  Boat #2 is a true victim, safe if plugged into shore power, potentially damaged if not.

All fault currents are always opportunistic.  They simply follow the rules of physics to find the “path of least resistance” back home.

The rate at which metal loss occurs is proportional to the voltage involved and to the many Ohms Law factors that determine the magnitude of accompanying current flow.  At its worst case, this process can sink a boat astonishingly quickly (a matter of hours/days), because with large uni-directional electrolytic currents (electroplating), metal mass can erode away from the anodic terminal(s) very quickly.   The part that gives up metal mass will ultimately suffer structural failure if the process is not interrupted.  If it happens to a thruhull on a boat in the water, the boat will sink.   All underwater metals – propellors, rudders, struts, trim tabs and radio ground planes – can be effectively “dissolved” by these stray electrolysis currents.

The best articles I know of for an understanding of this topic are by Capt. David Rifkin, who has good reference articles on his website, here: http://www.qualitymarineservices.net/corrosion.html.

Nigel Calder, Ed Sherman of ABYC and Steve d’Antonio have also written about these phenomena,  mostly in fee-based subscription publications like Professional Boatbuilder and Passagmaker Magazine, or in their own for-fee publications.

RAPID ZINC WASTING:

Like all metal corrosion, zinc wasting is a form of electro-chemical corrosion, always due to DC currents.  Electrical measurements of the basin occupied by the boat would be necessary to determine which mix of stray currents are present at any given time, but zinc wasting is a DC phenomena, and by far most commonly, a galvanic phenomena.

Boat owners can do their own basin-water testing, but it is not a process I would recommend for the electrical layman.   An understanding of the theory of these types of faults, understanding the probes and tools that are necessary, and the skills to correctly interpret test results are necessary.  This can be quite confounding, even to experts.

Boat owners that experience corrosion issues would be better served to hire an ABYC CORROSION-CERTIFIED MARINE ELECTRICIAN.  That said, skilled and knowledgeable boat owners can do their own DC testing with a silver/silver chloride half-cell.  High quality Digital Voltmeters (DVM) can detect AC ground fault currents, DC galvanic currents and electrolysis currents, but detection and evaluation is highly specific and sensitive to the placement of the measurement electrodes, quality of the test equipment, and conductivity of the surrounding basin water.

By the time a layman has bought the tools, learned to use them, and learned to interpret the results, said layman would be better off financially in hiring a professional who could provide the diagnosis and remediation recommendations as a one-time service.

WHAT THE BOAT OWNER CAN DO: DC ELECTROLYSIS CURRENTS:

  1. Diligently avoid having DC wiring wetted or submerged in bilge water.
  2. Never use the boat’s bonding system as a B- “neutral” return circuit for DC attachments.
  3. Avoid facilities (marinas, municipal or private docks, boatyards, etc) where the infrastructure appears to be poorly maintained.
  4. Be alert in marinas located in industrial neighborhoods where ground fault currents from shore sources may be more likely; check with the dockmaster for known issues in the basin.
  5. Avoid facilities with numerous boats that are in a poor state of maintenance and repair.

CHEMISTRY OF GALVANIC COUPLES:

CHEMICAL REACTION AT THE ANODE:

Oxidation occurs with the release of electrons and the simultaneous shedding of positively charged metal atoms which detach from the surface of the metal.  These particles enter the electrolyte solution as positively charged ions.  Chemically:

Fe → Fe++ + 2e (example with Iron);

Zn → Zn++ + 2e (example with Zinc);

Pb → Pb++ + 2e (example with lead).

CHEMICAL REACTION AT THE CATHODE:

Free electrons reach the cathode and react with hydrogen ions in the electrolyte.  Hydrogen bubbles will often form on the cathode; clearly visible in lead/acid batteries.  Chemically:

2H+ + 2e → 2H

CHEMICAL REACTION IN THE ELECTROLYTE:

If acid is not available, water itself will break down (dissociate) to generate hydrogen ions (H+).  The specific chemistry here depends on the composition of the electrolyte.  Assuming water, water dissociates, forming free hydrogen and hydroxyl ions:

H2O ⇌ H+ + OH

Then, metal ions combine to form metallic oxide, which is the corrosion product:

Fe++ + 2(OH) → Fe(OH)2, or

Zn++ + 2(OH) → Zn(OH)2, or

Pb++ + 2(OH) → Pb(OH)2

AC Electrical System

AC System Overview:

Note: An electrical diagram of Sanctuary’s AC Power Distribution  System as described in this article is located here (Adobe Portable Document Facility (.pdf) file): 20161022_ac_electrical_distribution_system.

The ship is wired to operate from two single-phase, grounded-neutral, 120VAC, 30A shore circuits originating in a dockside shore power system.  Neither the shore grounded (white) neutral conductor (white) nor the ungrounded energized conductor (black) is connected to the safety ground (green) aboard the ship (ABYC E11, 11.17.1.2).

The ship’s AC safety ground (green) and DC negative buss (black) are connected together in the engine room (ABYC E11, 11.5.4.7.1 and subs).

The ship is fit with a ProMariner® Prosafe-1™ galvanic isolator.  AT the time of it’s installation, the device complied with the then requirements of ABYC A28, 28.13 (now obsolete). The diode pack of the device is installed in series with the ship’s incoming green safety ground wire. The isolator and its control module are located in the ship’s electrical closet. The enumerator/monitor is mounted at the ship’s electrical control center located stbd, in the companionway to the vee berth.

Note: the ProMariner® galvanic isolator control module was disconnected in May, 2016.  The design of the enumerator/control module places a ground fault on the incoming shore power circuit.  That ground fault is used by the device to test the ship’s safety ground wire for continuity.  That ground fault can trip shore power ground fault sensors.  Disconnecting the enumerator does not affect the purpose or operation of the diode pack itself, but it does defeat the self-checking feature of the OEM design of the enumerator/monitor.

The ship is fit with two 120VAC, 30 Amp SmartPlug® marine shore power inlet connections (ABYC E11, 11.6.3.1.2 and subs). “Shore 1” corresponds to a Newmar® ACDC-1™ “house” AC Load Center (identified with the numeral “1”). AC branch circuit breakers (ABYC E11, 11.10.1.5 and subs) serving the genset battery charger, refrigerator/freezer, 120VAC water heater, inverter/charger and several house utility outlet circuits are installed. “Shore 2” corresponds to a Weems and Plath® “heat pump” Load Center (identified with the numeral “2”). AC branch circuit breakers (ABYC E11, 11.10.1.5 and subs) serving two reverse cycle heat pumps and a raw water circulator pump (“air conditioner” units) are installed.  It is not necessary to connect both shore power circuits in order to use either one; each is completely independent of the other.

The ”AC Master Breaker” on the NewMar® “house” AC Load Center and the ”Master Breaker” on the Weems and Plath® “heat pump” Load Center function as shore power disconnect switches (ABYC E11, 11.10.2.6 and subs and 11.17.1.1 1). These double-pole breakers isolate their respective AC shore power circuits from the ship’s on-board AC distribution system. The 120V energized current-carrying (“hot”) buss on the AC side of the NewMar® Load Center has been modified from its OEM configuration. Breakers 1-4 on the NewMar® panel are fed by shore line-in or generator power. Breakers 5-8 on the NewMar® panel are fed by the onboard inverter/charger as described later in this document.

Footnotes:

  1. NoteThis section of E11 was upgraded to include ELCI in the July, 2012 release of the standard. Sanctuary complies with the July, 2009, release, E11, 11.7.2.2.1.1. Sanctuary is not fit with ELCI at this time.

The ship is fit with an ONAN® MDJE™ onboard generator. The generator is powered by aa Onan 2-cylinder, 4-cycle diesel engine. Diesel fuel for the generator engine is drawn from the ship’s onboard fuel tanks. The generator has a 2-stage (fresh water with heat exchanger) cooling system. The generator is rated for continuous operation at 7.5kW, 60Hz. Generator operation (glow plugs, start/stop) is controlled by two rocker switches mounted at the ship’s electrical control center.  The generator and its diesel engine are mechanical devices that require periodic preventive maintenance. Refer to the Onan manual for maintenance schedules.  The generator’s AC output is wired in a 240VAC configuration. The generator starter motor is fed through a BlueSea Systems ML-RBS remotely operated DC disconnect switch (ABYC E11, 11.6.1.2.1, 11.6.1.2.2).

The ship is fit with a Blue Systems® p/n 9093 manual Generator Transfer Switch. The GTS transfers the ship’s AC distribution load centers to either the shore power inlets or the onboard generator. When shore power is available, and the load center disconnect switches are set “on,” the corresponding green “Shore Power” LED on the GTS operator’s panel becomes illuminated. The GTS is of the break-before-make design. This prevents simultaneous cross-connection of incoming shore power source(s) and the ship’s generator (ABYC E11, 11.5.5.6). This allows the generator to be started and run for servicing while the ship is simultaneously connected to energized (live) shore power connections.

Shown following is the wiring layout of the GTS as installed. Shore power service cords feed “Source 1” and “Source 2.” The 240V onboard Generator feeds “Source 3.” The NewMar® AC Load Center for “house” loads is connected as “Load 1;” the Weems & Plath® Load Center for  heat pumps is connected as “Load 2.” The GTS is shown in the “Shore” position. If the generator is “running,” the green “Generator” LED on the GTS operator’s panel becomes illuminated. If both shore and generator power are available at the same time, both sets of LEDs will be illuminated.

gts

The ship is fit with a Magnum® MS2012™ Pure Sine Wave Inverter/Charger.

  1. When either shore or generator power is available, the device operates in “Passthru” mode to forward AC power to utility outlets via circuit breakers 5–8 on the NewMar® AC Load Center and to simultaneously charge the ship’s battery bank. When shore or generator power is not available, the device operates in “Invert” mode as the AC power source for circuits 5-8. The device automatically switches between its “passthru” and “invert” modes as availability of shore or generator power changes.
  2. The neutral buss for branch circuits powered by the inverter/charger is isolated from the neutral buss for branch circuits powered solely by shore or generator power. Magnum requires this separation, which they base on ABYC A31, 31.6.7.3.1. However, AC neutrals are defined to be “grounded conductors.” Therefore, this ABYC reference seems obscure, since it refers to separation of “ungrounded conductors.”
  3. Configuration of the operational status of the inverter/charger is manually selectable via the ME-RCtm Remote Control mounted at the ship’s electrical control center area.
  4. The “fault” lamp on the ME-RCtm Remote Control indicates a problem that prevents normal operation of the inverter/charger. The two most common faults are spike voltages and out-of-tolerance frequency deviations. These faults sometimes occur when the ship is operating on the generator as its AC power source and heavy loads cycle “on” and “off.” These “faults” can be manually cleared by recycling DC power via the inverter/charger battery disconnect switch in the engine room space.
  5. DC electrical energy to power the inverter/charger originates from either 1) the propulsion engine’s alternator (supplemented by the battery bank), if the ship’s propulsion engine is running, or 2) solely by the ship’s battery bank, if the ship’s propulsion engine is not running. In the absence of shore or generator power, the DC ampere-hour (aHr) capacity of the ship’s battery bank can be conserved by discontinuing use of the Inverter/Charger and its attached AC loads.

Finally, the ship is fit with a 700-watt Xantrex® Modified Sine Wave (MSW) utility Inverter. This inverter is an alternative AC power source that can feed a utility outlet power strip on the ship’s salon nav station. This inverter is a stand-alone device that is not integrated into the ship’s AC distribution system. It is available to power the satellite TV receiver/DVR and the TV. DC energy supply for this inverter is as described above. The device mounted on the aft bulkhead of the ship’s standing closet.

DEFINITION: “Secured State” of the Ship’s AC Electrical System

Aboard Sanctuary, a “secured state” for the AC Electrical System is defined to exist when all of the following conditions exist:

  1. All individual AC house circuit breakers on the NewMar® AC Load Center are in the “off” position,
  2. All individual heat pump circuit breakers on the Weems & Plath® Load Center are in the “off” position,
  3. The “AC Master Breaker” on the Newmar® AC Load Center and the “Master Breaker” on the Weems and Plath® Load Center are both in the “off” position,
  4. The generator transfer switch is in the “off” position,
  5. Shore power service cords are disconnected and stored aboard,
  6. The generator is not running,
  7. DC power to the Xantrex® MSW Inverter is discontinued via it’s disconnect breaker, located in the electrical closet,
  8. DC power to the Magnum® MS2012tm system-integrated Inverter/Charger is discontinue via the DC rotary disconnect switch located in the engine room, stbd bulkhead.

References to ABYC E11 contained in this document:

All references to ABYC E11, AC AND DC ELECTRICAL SYSTEMS ON BOATS, are to the July, 2012, release of the standard unless otherwise noted.

DC Electrical System

DC System Overview:

Note: An electrical diagram of Sanctuary’s DC System as described in this article is located here (Adobe Portable Document Facility (.pdf) file): 20161022_dc_electrical_distribution_system.

The ship’s DC electrical system is a “negative ground” system. (ABYC E11, 11.4.24 and 11.5.4.3)

The DC system is of the “ungrounded” system design (ABYC E11, 11.4 and E11, 11.5.4.3).

The ship’s AC safety ground (green) is bonded to the Ship’s DC negative buss (black) in the engine room. (ABYC E11, 11.5.4.7.1.2)

DC energy for the “house” and “engine start” services originates in a single battery bank comprised of six, 6 volt, flooded deep cycle “Golf Cart” batteries. Over-current protection (OCP) for the battery bank is provided by BlueSea Systems®, Type MRBF™, 200A fuses. (ABYC E11, 11.10.1.1.1 and 11.10.1.2 and subs).  Details of that design decision are discussed in my article, “Battery Bank: Separate vs Combined,” on this site.

A single Group 27 start-service battery is used to start the ship’s generator. A BlueSea Systems® model ML-RBS™ remotely operated disconnect switch, with p/n 2145 remote switch, is fit in the generator’s battery feed circuit.

The ship’s Main DC Disconnect Switch (ABYC E11, 11.6.1.2 and subs) for the DC feed to the NewMar® Load Center and other attached DC loads is located in the engine room, stbd bulkhead, above the ship’s battery bank.

The ship’s OEM NewMar® Model ACDC-1™ Branch Circuit Load Center/Distribution Panel is located to stbd in the forward companionway to the vee berth. Because there is a combined battery bank for house and start functions, the OEM Battery Selector Switch is discontinued and removed. The left half of the Newmar® Load Center serves as the DC Load Attachment Center for many DC attachments, including:

  • navigation, anchor lights and deck lights,
  • LPG safety shutoff valve,
  • refrigerator/freezer DC feed,
  • house drinking water pump,
  • raw water wash-down pump,
  • DC lighting,
  • salon VHF radio,
  • windshield wipers and horn,
  • toilet macerator, and
  • stereo.

This panel provides over-current protection for attached loads via circuit breakers. (ABYC E11, 11.10.1.5)

The ship is fit with a Blue Sea® System WeatherDeck™ DC Circuit Distribution Panel, located stbd on the flybridge. It is the primary load attachment center for the ship’s navigation instruments, including:

  • chart plotters (including Radar),
  • auto pilot controller,
  • VHF radio,
  • depth sounder,
  • under-console AIS receiver and wi-fi multiplexor, and
  • flybridge DC utility outlets.

This panel provides over-current protection for attached loads via circuit breakers. (ABYC E11, 11.10.1.5)

The ship is fit with a Magnum® MS2012™, Pure Sine Wave (PSW) Inverter/Charger. DC over-current protection is provided by a 300A Class “T” fuse (ABYC A31, 31.5.2.4.4 and subs; 31.6.4.1). The DC Disconnect Switch for the inverter/charger is located in the engine room, stbd bulkhead, above the ship’s battery bank. (ABYC E11, 11.6.1.2 and subs)

DC Power for some ship equipment attaches to the vessel’s DC Distribution System independently of the NewMar® load center. These attachments have individual over-current protection. These attachments include:

Attachment

Location of Attachment’s
Disconnect Switch
or OCP

(ABYC E11, 11.6.1.2 and subs, 11.10 and subs)

1 propulsion engine
a) starter motor feed and
b) control, instrumentation and instrument lighting circuits
fwd surface of galley drawer cabinet, floor level, aft of stbd salon door; switch is accessible without opening the engine room. This circuit is without OCP. (ABYC E11, 11.10.1.1.1, Exception 1)
2 Lewmar® H900 windlass tagged; on vessel’s electrical control center
3 Dickson® stern thruster Inline fuses, respective operator’s panel
4 Garmin® GHP10tm autopilot system tagged; engine room, stbd, mid-ships, above battery boxes
5 Magnum® MS2012tm, 2kW, PSW inverter/charger tagged; engine room, stbd, mid-ships, above battery boxes
6 ICOM® 706MKIIGtm Amateur Radio Transceiver Inline fuses, electrical closet, NewMar® B+ buss
7 bilge pump tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
8 high bilge alarm tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
9 shower sump pump tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
10 SirenMarine® Spritetm Boat Monitor inline fuse, electrical closet
11 salon +12VDC utility outlet. inline fuse, electrical closet

Battery State-of-Charge Status & Monitoring:

The ship is fit with six, Duracelltm, EGC-2, Golf Cart batteries.  These batteries have a 20-hour Ampere Hour (aHr) rating of 230 aHr and a Reserve Capacity rating of 448 minutes. These batteries are manufactured by East Penn Manufacturing® and are sourced from Sam’s Club. The batteries are combined in a series/parallel configuration to provide 12VDC.

The overall total rated ampere hour capacity of the ship’s battery bank is 690 aHr.

A Magnum® ME-RCtm Remote Control, and a Magnum® ME-BMKtm Battery Monitor Kit, are installed to monitor the ship’s DC electrical operating parameters and the state-of-charge status of the ship’s battery bank.

“Standard operating practice” aboard ship is to adhere to the “mid-capacity rule,” which states that lead-acid batteries (flooded wet cells, AGM or Gel) should not be discharged beyond 50% of their rated capacity and that discharging a lead-acid battery in excess of 50% of its capacity shortens its service life. Aboard ship, the ME-RCtm/ME-BMKtm Battery Monitor displays the “State-of-Charge” of the battery bank. Less overall depth-of-discharge is better.

The approximate eight-hour overnight (summer hours-of-daylight) DC system electrical consumption (refrigeration, anchor lighting, evening TV watching, computer use, etc.) aboard ship is 125 – 150 aHr. The approximate eleven-hour overnight (winter hours-of-daylight) DC electrical consumption is 150 – 200 aHr.

Battery Charging:

Shore Power/Onboard Generator:
When the ship is receiving AC power from either 1) shore power or 2) from the ship’s onboard generator, the ship’s battery bank is charged by the Magnum® MS2012tm 2kW Inverter/Charger 1.

Under way:
The ship’s engine alternator charging system consists of a single, 12-volt, 110 amp, Balmar® high-output alternator, model 712110, with a Balmar® MC-614tm external regulator. The external regulator receives its DC input power supply from the propulsion engine’s starter solenoid 2.

DC Distribution Wiring: 3

  1. Three attachments originate at the positive battery post of the ship’s battery bank:
    1. Attachment 1 is BlueSea Systems®, Type MRBFtm Fuse Block, p/n 2151, fit to the designated positive output terminal. (ABYC E-11, 11.12.1.2). This fuse block is fit with two, 200A, Type MRBF fuses.
      • Fuse 1 feeds the ship’s Main DC Power Disconnect Switch 4, a Blue Sea® Systems, p/n 6006, rotary switch, via a 2-0 AWG red wire. A 2-0 red wire continues from the disconnect switch to a BlueSea® Systems, 150A, Type ANL fuse block. The house DC feed is a 2-0 AWG red wire from the ANL fuse block to a 600 amp, 4-post terminal block. The terminal block is the ship’s Main DC Power Distribution Buss.
      • Fuse 2 connects the ship’s Balmar 110A alternator to the battery bank, via a #6 AWG red wire.
    2. Attachment 2 is a 2-0 AWG red wire that feeds the ship’s propulsion engine starter solenoid through a BlueSea System®, p/n 6006, disconnect switch located in the salon, stbd, beneath the fold-down table. This circuit is not overload protected (ref: ABYC E-11, 11.12.1.2).
      • The propulsion engine starter solenoid is the DC system attachment point of the engine’s DC sensors, controls, instrumentation and malfunction alarms, and the external Balmar® MC-614tm voltage regulator which energizes the on-engine Balmar® alternator unit.
    3. Attachment 3 is a 2-0 AWG red wire (ABYC, E-11, 11.12.1.1.1) that feeds the ship’s inverter/charger through a Class “T” fuse block fit with a 300 Amp, Class “T” fuse. A 2-0 AWG red wire continues from the Class “T” fuse block to a BlueSea Systems® p/n 6003e disconnect switch. From the disconnect switch, a 2-0 AWG red wire feeds the Inverter/Charger.
  2. House loads aboard ship are fed from the Main DC Power Distribution Buss.
    1. A 1-0 AWG red wire runs from the terminal block to a terminal block in the electrical closet. This is the DC attachment point for the ship’s bilge pump, high bilge alarm, NewMar® DC Load Center, BlueSea Systems® WeatherDecktm Load Center, Xantrex® Inverter and shower sump pump.
    2. A 1-0 AWG red wire from the terminal block feeds the vessel’s 70 amp windlass circuit breaker. From the circuit breaker, a 1-0 AWG red wire joins in the electrical locker with an OEM 38mm2 red wire to feed the vessel’s windlass contactor, located forward, in the chain locker overhead.
    3. A #8 AWG red wire feeds a 40 amp circuit breaker which feeds the Garmin® GHP10tm autopilot system.
    4. A fused attachment feeds the Magnum® ME-BMKtm battery monitor module.

Footnotes:

  1. Note: Setup options for the Magnum Inverter/Charger and ME-RC Remote Control are found in the MS-Word document entitled “Magnum_Remote_Control_Setup_Customization.doc.”
  2. Note: Setup options for the Balmar ARS-5 external regulator are found in the MS-Word document entitled: “Balmar_ARS-5_Setup_Customization.doc.”
  3. Note: the written description contained in this document is diagrammed in the document entitled ”DC_Electrical_Distribution_System.PDF.” Details of electrical system attachment points are documented in a table entitled “DC_Branch_Circuit_Functions.XLS.”
  4. WARNING: This  disconnect switch removes power from the bilge pump, sump pump and high-bilge alarm circuits. Therefore, it is to be used only for attended servicing of the electrical system. It is not intended for long-term disconnection of the battery bank while the boat is in the water!

Balmar® 110A High Output Alternator:

An alternator is a “self-limiting” device (ABYC E11, 11.4.26). That is, “a device whose maximum output is restricted to a specified value by its magnetic or electrical characteristics.” What that means is that an alternator can only produce just so much output current – in this case, 110A – regardless of how much drive is applied to its field winding. If the alternator fails, output generally stops. For self-limiting devices, a fuse is not “required” by ABYC E11, 11.10.1.1.2, but it is a reasonable precaution.

Aboard Sanctuary, the alternator connection to the ship’s DC system is protected by a 200A, Type MRBF, fuse. Two Hundred amps may seem too large a rating. The maximum rated output of the alternator is 110A. The ampacity of the #6 AWG conductor, derated because it’s in an engine space, operating in a 12V system with a 3% voltage drop and 105ºC insulation is 80A, and substantially higher for the 10% voltage drop case. So it may appear there is no scenario where the 200A fuse would protect the alternator connection wiring.

It’s very important to understand that we DO NOT want that fuse to open in the absence of a true, sustained over-current situation. If the fuse were to open while the alternator was operating in its designed power output range, the effect would be to instantly disconnect the electrical load (the battery bank) from the alternator. The magnetic field inside the machine would instantly collapse, creating a very high voltage “spike” inside the windings of the machine. That spike would almost certainly destroy the alternator’s internal diodes and render the device inoperable.

Alternators contain solid state full-wave rectifier diode pacs. Internal diodes prevent DC power from flowing backwards through the alternator when the engine is not in operation. If diodes fail in a welded-closed state – “shorted,” or “short circuited” – the result would create a direct path from the battery to ground via the alternator’s stator windings. The purpose of this fuse is to protect against that true over-current condition. In the case of a shorted diode, the 200A fuse protects the charging wire from becoming overloaded and, thus, a fire risk.

Magnum® MS2012tm Pure Sine Wave Inverter/Charger:

The positive and negative DC wires for the ship’s PSW Inverter/Charger are 2-0 AWG BC5W2 boat cable. Round-trip distance from battery-to-device and return is approximately 10′. The rated ampacity for 2-0 AWG primary wire inside an engine space is 280 Amps DC (ref: http://marinco.com/page/conductor-sizes). Magnum’s MS2012tm specs state that maximum battery charging current is 100 Amps DC, and maximum full load inverter draw at rated AC output load is 225 Amps DC. Optional configuration options are available via the ME-RCtm Remote Control to limit Inverter/Charger operating currents.

DC Return (DC “Ground”):

There are two terminal blocks in the ship’s engine room that comprise the ship’s DC return circuit to the main battery bank. The DC branch circuits and AC Safety Ground are all collected on a terminal block located fwd in the engine compartment, at deck level, starboard. This terminal block is connected to the main DC negative buss terminal block via a 1-0 AWG black wire.

The main DC negative buss is a Blue Sea® Systems 600A terminal block located amidships in the engine room, starboard, above the battery boxes. Joined at the main DC negative buss terminal block are:

  1. the consolidated branch circuit return terminal block, via a 1-0 AWG black wire,
  2. the Magnum® MS2012tm PSW Inverter/Charger, via a 2-0 AWG black wire,
  3. the propulsion engine block, via a 2-0 AWG black wire, (returns the Starter Motor circuit and the Propulsion Engine DC controls, instrumentation and instrument lighting), and
  4. attachment to the battery monitor’s 50mV, 500A measurement shunt, via a 2-0 AWG black wire.

The return connection to the negative terminal of the main battery bank is made from the battery monitor’s 50mV, 500A shunt, via a 2-0 AWG black wire.